Answering my own question here:
To get the Custom Floating HTML Prompt to appear centred in whichever display currently contains the frontmost app, you just insert the following action prior to the Custom Floating HTML Prompt action:
Answering my own question here:
To get the Custom Floating HTML Prompt to appear centred in whichever display currently contains the frontmost app, you just insert the following action prior to the Custom Floating HTML Prompt action: