How do I access the user selected text within an element ui input?

I want to access the text within an ElInput component via Javascript in Electron. According to mozilla it is impossible to access information within an html input or textfield via window.getSelection.

I unsuccessfully tried to access the selection with the proposed selectionStart

const input = document.getElementById("input")
alert(input.selectionStart.toString)

Given that this doesn't work, what do I have to do, to get the text in my selection within my el-input?