solution by: https://stackoverflow.com/questions/45890587/pyqt5-using-webengineview-like-a-widget

MainWindow.py

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'MainWindow.ui'
#
# Created by: PyQt5 UI code generator 5.6
#
# WARNING! All changes made in this file will be lost!

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(824, 728)
        MainWindow.setMinimumSize(QtCore.QSize(250, 150))
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setEnabled(True)
        self.centralwidget.setMaximumSize(QtCore.QSize(1218, 836))
        self.centralwidget.setLayoutDirection(QtCore.Qt.LeftToRight)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayout = QtWidgets.QGridLayout(self.centralwidget)
        self.gridLayout.setObjectName("gridLayout")
        self.downloadButton = QtWidgets.QPushButton(self.centralwidget)
        self.downloadButton.setObjectName("downloadButton")
        self.gridLayout.addWidget(self.downloadButton, 2, 0, 1, 2)
        self.urlInput = QtWidgets.QLineEdit(self.centralwidget)
        font = QtGui.QFont()
        font.setPointSize(10)
        self.urlInput.setFont(font)
        self.urlInput.setClearButtonEnabled(False)
        self.urlInput.setObjectName("urlInput")
        self.gridLayout.addWidget(self.urlInput, 0, 0, 1, 1)
        self.setURL = QtWidgets.QPushButton(self.centralwidget)
        self.setURL.setObjectName("setURL")
        self.gridLayout.addWidget(self.setURL, 0, 1, 1, 1)
        spacerItem = QtWidgets.QSpacerItem(20, 40, QtWidgets.QSizePolicy.Minimum, QtWidgets.QSizePolicy.Expanding)
        #self.gridLayout.addItem(spacerItem, 1, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 824, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "Youtube Downloader"))
        self.downloadButton.setText(_translate("MainWindow", "Download Youtube Video"))
        self.urlInput.setText(_translate("MainWindow", "Enter URL here (include http://www)"))
        self.setURL.setText(_translate("MainWindow", "Go to entered URL"))

MainWindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>824</width>
    <height>728</height>
   </rect>
  </property>
  <property name="minimumSize">
   <size>
    <width>250</width>
    <height>150</height>
   </size>
  </property>
  <property name="windowTitle">
   <string>Youtube Downloader</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <property name="enabled">
    <bool>true</bool>
   </property>
   <property name="maximumSize">
    <size>
     <width>1218</width>
     <height>836</height>
    </size>
   </property>
   <property name="layoutDirection">
    <enum>Qt::LeftToRight</enum>
   </property>
   <layout class="QGridLayout" name="gridLayout">
    <item row="2" column="0" colspan="2">
     <widget class="QPushButton" name="downloadButton">
      <property name="text">
       <string>Download Youtube Video</string>
      </property>
     </widget>
    </item>
    <item row="0" column="0">
     <widget class="QLineEdit" name="urlInput">
      <property name="font">
       <font>
        <pointsize>10</pointsize>
       </font>
      </property>
      <property name="text">
       <string>Enter URL here (include http://www)</string>
      </property>
      <property name="clearButtonEnabled">
       <bool>false</bool>
      </property>
     </widget>
    </item>
    <item row="0" column="1">
     <widget class="QPushButton" name="setURL">
      <property name="text">
       <string>Go to entered URL</string>
      </property>
     </widget>
    </item>
    <item row="1" column="0">
     <spacer name="verticalSpacer">
      <property name="orientation">
       <enum>Qt::Vertical</enum>
      </property>
      <property name="sizeHint" stdset="0">
       <size>
        <width>20</width>
        <height>40</height>
       </size>
      </property>
     </spacer>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>824</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
 </widget>
 <resources/>
 <connections/>
</ui>

ProcessStarter.py

import sys
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *
from PyQt5 import QtWidgets, QtCore, QtGui, QtTest
from PyQt5 import QtWebEngine,QtWebEngineWidgets, QtWebEngineCore
from PyQt5.QtWebEngineWidgets import QWebEngineView
import os, subprocess, youtube_dl, MainWindow
from MainWindow import *

class App(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        self.setUpStuff()

    def setUpStuff(self):
        #self.engineView = QWebEngineView
        #self.engineView.load(QWebEngineView, QtCore.QUrl("http://www.Youtube.com"))
        #self.webView.setEnabled(True)
        #self.webView.setObjectName("webView")
        #self.webView.setBaseSize(30, 30)

        self.webView = QWebEngineView()
        self.webView.load(QUrl('http://youtube.com'))
        self.webView.setBaseSize(500,500)
        self.webView.setVisible(True)
        self.getActualURL()

        self.gridLayout.addWidget(self.webView, 1, 0, 1, 2)

        #ui.downloadButton.setEnabled(False)
        #ui.setURL.setEnabled(False)
        self.downloadButton.clicked.connect(self.youtubeDownload)
        self.setURL.clicked.connect(self.setURLm)
        #self.getActualURL()
        #self.webView.installEventFilter(self)

    #def event(self,obj):
        #if obj == self.webView:
            #if(self.webView.hasFocus() == True):
                #self.getActualURL()
                #self.setURLm

        #return False

    def getActualURL(self):
        print("getactualurl")
        i = self.webView.url()
        i = str(i)
        i = i[19:]
        ilen = i.__len__()
        url = i[:ilen - 2]
        print(url)
        #self.urlInput.setText(url)


    @pyqtSlot()
    def youtubeDownload(self):
        print("ytdownload")
        try:
            with youtube_dl.YoutubeDL({}) as ydl:
                login = os.getlogin()
                path = "C:/Users/"+login+"/Downloads/Youtube_Downloader"
                if not os.path.exists(path):
                    os.makedirs(path)
                os.chdir(path)
                ydl.download([str(self.urlInput.displayText())])
        except:
            msgBox = QMessageBox()
            msgBox.setText("Error:")
            msgBox.setInformativeText("An unknown error occurred. Ensure video link is correct")
            msgBox.setStandardButtons(QMessageBox.Ok)
            msgBox.setDefaultButton(QMessageBox.Ok)
            msgBox.exec_()

    @pyqtSlot()
    def setURLm(self):
        print("seturlm")
        try:
            url = self.urlInput.displayText()
            i7 = url[:7]
            i8 = url[:8]
            if (i7 == "http://" or i8 == "https://"):
                pass
            else:
                url = "http://"+url
            print(url)

            self.webView.load(QUrl(url))
        except:
            pass

app = QApplication(sys.argv)
l = App()
l.show()

sys.exit(app.exec_())

pyuic5.bat

@C:/Users/Michael/Anaconda3\python.exe -m PyQt5.uic.pyuic %1 %2 %3 %4 %5 %6 %7 %8 %9

YoutubeDownloader.md

YoutubeDownloader

PyQt5 Youtube Downloader