Notes/Domino 4 and 5 Forum

I would like to know how to display a message box in the Formula language when @username matches an existing field on the form? We are using @If to check this condition but the @Prompt formula doesn't display anything.

my code is as follows:

@If(@UserName=doc.Lastupdatedby;@Prompt([OK];"You cannot approve this form";"");
counter := counter + 1;
match := "True"
@If(@UserName=doc.Lastupdatedby;exit;"");

notes:
doc.updatedby refers to a field on the form
counter is an Integer variable
match is a String variable