Here’s some advice I received years ago for this very situation: if you’re struggling to piss due to stage fright, imagine pissing on the head of someone you hate. Really imagine it. Imagine micturating all over their hair (or their scalp), the aghast expression, then them spluttering while trying to mouth ‘What the fuck?’ it does work. I don’t know why it works - and I most definitely don’t want to know - but it does work.

You’re all most welcome :)