Hello, this is what has worked to help my PDA daughter ...
First thing is to stop talking about using the toilet, don't mention it or refer to it.
Allow the accidents for a bit, at least releasing the urine gets it out.
Think carefully about which PDA strategies your child has already responded well to in regards to other demands, and gently try the best-success approach around toileting. Back away immediately if she gets more rather than less anxious.
For example, if eating is a demand and you have found that distraction with the iPad or tv helps to reduce anxiety (eg she watches iPad whilst eating), then try to set up a couple of opportunities to have the iPad on the loo/potty. Be careful, if you present it too strongly or she thinks you've planned it, she will smell that it's a tactic and feel tricked. Instead, maybe you are sitting on the loo yourself watching the iPad, when your partner/friend brings her into the bathroom to start running her bath. No comments, no "oh look, mummy sometimes watches the iPad on the loo, do you want a turn?" None of that, avoid! Let her see you and ask to do it herself if she wants to.
For us, we found humour was the best strategy, or role play.
Humour - the toilet seat starts talking to her in a funny accent when she sits on it. The toilet paper takes on a life of its own and somehow gets itself wrapped around your head whilst you're trying to wrestle it off.
Role play - my child uses role play a lot to help her, so we use the characters she has created for herself.
Before you leave the house for a trip out somewhere ... you quietly start thinking aloud in her earshot ... "hmm, we're going to be out of the house for a while, and there's isn't a toilet at the park, I've been to the loo just now so I think I'll be ok, but I know DD hasn't been for a couple of hours so I need to think of a solution for that ... but I'm stuck, I haven't got any ideas ..." Wait, see if she comes up with the idea that she could go to the loo now before you leave. You can look happy and agree that's a good plan, but don't go crazy with praise etc.