F4 function (drop down list) with limited values/selections

Hi Everyone,

I would like to ask if this is possible, and if yes, then how:

I would like create a selection screen where the user will select a certain UNAME from the drop down list. This selection field will have possible values taken from SU01 user names. But here's the twist; the selection field should not allow users to type on it or select a blank value. The user is obliged to select from the drop down list. Is this possible? If yes, please tell me how to do this or you may share some links for me to follow.

Thank you very much in advance.

Regards,

RE