MS-Access Beginner Replace Textbox Caption With Combo Box Selection on Button Press

I'm a complete beginner, just trying out/looking into Databases as more of a hobby, and think Microsoft Access would be an easy way to start with the whole front-end forms and back-end combination. I just have a form with a combo box [UserNameEntry] that you can select a value from a table 'Users' in a 'Alias' field and then just press a button [Command27], and another text box will have its text replaced by that string. This is more of a test/precursor before me wanting to set it to get the record matching the 'Alias' value selected in the combo box, then checking if another field in that same record matches a particular value. I don't mind if any help is in the expression, macro or VBA builder, all is good to learn. I tried the following expression in the On Click event of the button [Command27] (and many other alternatives) but haven't had much luck:

=[Command27].[Caption]="[Users]![Alias]=" & "[UserNameEntry]"

Any help would be appreciated!