Advice for plasterboarding under window

[diynewbie123]

I have a window insulation issue that I am seeking advice on.
I live in a flat, and all my window frames are floor to ceiling. the bottome half of the window frame is a Upvc material that offers little (or no insulation) The top half is the glass pane.

I had an idea of affixing thermal plasterboard internally to the bottom uPVC half of the windows.
Does anyone have an idea of the best way to stick the plasterboard to the uPVC (without nails and screws obviously)
Will normal adhesive do the trick, or is there a better way?

Thanks