Question : Validation Rule for TextBox in a Form

I have a Form where I can enter the "UserID" in a TextBox so the related info will be displayed.
I want to add Validation Rule to this TextBox. If the UserID entered is not in the Field [UserID] of the Table [tblUser], a Validation Text will pop out.
I tried a few times to use expression builder, but everytime got error. Could someone help me on that?
Thanks!

Answer : Validation Rule for TextBox in a Form

yes. with combo you can either select from the list or type a value