how to execute a get request in behaviour and set multiline textfield value as json reponse?

[rakesh rocckz]

I created a rest endpoint it works fine. I have a multiline text field and a list with a single select.

when I select the something in the single select list it should execute a get request and show the JSON response in the multiline text custom field. is it possible?? please help me out. 

i have a script which basically does the ajax call it converts the multiline textfiled to dropdown by using "convertToMultiSelect" in the same way can i convert to texfield? is there anything which converts to asusual text??