How to use variable in Exec statement
See @DanGuzman comment above. It did the trick. I have to remove the quotes and N from the EXEC statement for my variable. Credit goes to Dan Guzman. 

EXEC msdb.dbo.sp_add_jobserver @job_id=N'jb3510a9-d75f-4906-b9e3-en74b947d7ev', @server_name = @TargetServer