Hi!

I don't think the getDisplayValue takes any parameters. It will get the display value of the current record on the form:

- https://developer.servicenow.com/dev.do#!/reference/api/tokyo/server_legacy/c_GlideRecordAPI#r_Glide...

You will need to get the reference of the on_behalf_of and the get the value of the user_name:

g_form.getReference('on_behalf_of').getValue('user_name');

I hope it helps!

Pedro