@jaheerhattiwale : Yes I have tried this and logic is working but there are some issues. 

Found one issue/scenario.

[] Suppose I have loaded the catalog form with my name ( logged in with my id). So it will give me the response from the API for my id and we are storing the responses in one of the field. This is working fine.

But If I have loaded the form with my id and changing the "requested for" to some other name then , the drop down options are getting added. In the drop down field ( options available for the user which is loading the form+ Requested for user ( if changing to some other user name)).

[] Also currently the script is getting executed two times in backend. In logs the same things are getting printed two times at the same time with a difference if 2/3 secs.

Desired scenario :

If I am a loading the catalog form with my id , then it will show the API response for my id and same will be stored in drop down field.

But if I am changing the value of "Requested for" field , then the drop down field should only show the API response of the userid which is selected in "Requested for" field. It should clear out the choices for the previous user. Currently it is adding the choices.