tempvars show as null when used in a function!

tempvars variable does not show value when used in function!  

I set the variable in a login form to use throughout the project and that works fine!
TempVars.Add "log_usr", UserName

But when I try to use it in a function it shows NULL.

On a form I put this on a button! I'll use it on an autokeys macro later once it works!  
    DoCmd.OpenFunction Open_user_text()

    Function Open_user_text()


       

        MsgBox (" User =") & tempvars!UserName
 

    End Function



Thanks

Pete