Inserting text on click

I have a input text field on the stage and I want to insert text into that inputTF via a button. I can’t use something like

add.onRelease = function(){
   inputTF.htmlText = "hello world"

becase it replaces the user’s inputed text. I want to just insert text where there “flashingType-dealy” is located (my apologies I don’t know what it is called).

I think a starting point is using Selection


Selection.setSelection(this.tempBegin, this.tempEnd);
//what else? //