Get Email Id By Subject

Code.gs

function onOpen() {
  var Ui = SpreadsheetApp.getUi()
  var menu = Ui.createAddonMenu()
    .addItem("Get Message ID", "getMsgIds")
    .addToUi()
}
function getMsgIds() {
  var MsgSubject = Browser.inputBox("Enter the subject of the Email");
  if (!MsgSubject) {
    Browser.msgBox("oh oh, seems you forgot to provide the message subject, please try again. I\n\\n\\n\\n Thank you! \\n\\n Regards, l\n G")
  }
  if (MsgSubject) {
    var Threads = GmailApp.search('subject:' + '"' + MsgSubject + '"')
    for (i = 0; i < Threads.length; i++) {
      var MsgId = Threads[i].getId()
      var Subject = Threads[i].getFirstMessageSubject()
      Logger.log(`Subject: ${Subject}, MsgId: ${MsgId}`);
    }
  }
}