find where was the cursor inside a textbox

Hi! good afternoon everyone!!!
I'm trying to do a multi line textbox field and place a button that inserts a variables inside the textbox when people click it.
By the moment it works fine but I have only one problem and thats why I post here! =)
What I want: When i click on the button  i want to add the variable into the position of the cursor
What I have: When i click on the button the variable is inserted on the last part of the textbox
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My code for the variable's button:
textbox1.text= textbox1.text + "i add this to the text"; __________________________________________________________________
what's the problem?




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