The importance of coffee in this equation cannot be overstated. If you’re worried, try having a 12oz coffee plus lots of water an hour or two before your appointment. Don’t hold it in all day before the appt, you might hurt yourself. Just goose your waterworks with coffee and stay well hydrated before going in and you should be fine.
Also, doctors know about bashful bladders so just let them know you might be a minute. You may also be able to collect a sample at home if you discuss it with them first(don’t roll up to your doctor today with a cup of unasked-for pee). You’ve got options. Just when you’re in the moment, relax and let the coffee do the work.