Testing A Stored Procedure With Output Params In SQL Server Managment Studio

I have an SP like this (edited for brevity):

set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO

ALTER PROCEDURE [dbo].[TESTING_SP]
@Username MediumText,
@Password MediumText,
@UserKey int OUTPUT,
@RoleKey int OUTPUT,
@UserGroupKey int OUTPUT,

AS

BEGIN

SELECT
@UserKey = UserKey
FROM UserProfile
WHERE Username = @UserName
AND [Password] = @Password
END

I want to execute this sp in Managment Studio (MS) and see what is being returned but I'm getting this error:

Msg 201, Level 16, State 4, Procedure TESTING_SP, Line 0
Procedure 'TESTING_SP' expects parameter '@UserKey', which was not supplied.

How do I set up the output parameters and then select the values in MS for testing purposes?

Thanks a ton for helping a noob.