Setvalue() client script not working

Setvalue() client script not working

I have created a client script on the load the record(detail view page). I wanted to populate some default information in the single line field. for that I created the client script.
below is the script:
var field_obj = ZDK.Page.getField( 'Designation' );
field_obj.setValue('Test Value');

client script trigger detail:


Now when the detail page load it's show "unsupported action" in the log. I really do not know why this is not working.

if anyone something related to this please let me know in comments. Thanks!