Making a field in customers portal visible only for agents adding requests

[Artur Kesik]

In the default configuration, if we open the customers portal logined as agents, we see one additional field: Raise on behalf of. 

I need another field "Channel" or different custom field, shown only to the agents adding requests. Normal user shouldn't see it.

Is it possible to do without plugins? 

[Jobin Kuruvilla [Adaptavist]]

You can't do it without plugins.