How should this wall be plastered prior to built in wardrobes being fitted?

I recommend priming the wall with blue grit 2 coats, then once dry dot and dab plasterboard onto the primed surface with plasterboard adhesive . Allowing solid dabs on floor and ceiling line and any internal corners ..

If it’s a exposed brick wall you can dot and dab and stick plasterboard up to give it a good background. You could also fix 2x1 battens to the wall and then plasterboard onto them for a solid background aswell .

Get a plaster to the plastering and get the carpenter to do the carpentry fitted means fitted we can get over most things that’s the trade

If the wall that you are plastering too is already solid plaster then dot and dab is going to be potentially too thick and therefore provide a step in the line of the finished surface. It is also going to crack at the joint and if you use plasterboard against existing plaster it's going to be a very obvious straightish crack. I would hack back a little of the existing plaster to provide a key for the new plaster, then one coat bonding/scratch plaster followed by a skim coat to finish. This will provide a better quality finish and a solid background to fix the wardrobe too.