Create a message object to use with Gmail API so you don't have to find in countless Google searches.

gmail_create_message.py

import base64
import email.policy
from email.mime.text import MIMEText


def create_message(sender, to, subject, message_text):
    """
    Create an email message.

    :param sender: Sender's email (can use format '{name} <{email}>' to show custom sender name)
    :param to: Receiver's email
    :param subject: The email subject
    :param message_text: Email content (HTML supported)
    :return: Message body (to use with Gmail API)
    """
    message = MIMEText(message_text, 'html', policy=email.policy.default)
    message['to'] = to
    message['from'] = sender
    message['subject'] = subject
    raw_message = base64.urlsafe_b64encode(message.as_string().encode()).decode()
    return {
        'raw': raw_message
    }