If it's really a pcell (defined with pcDefinePCell), then it makes no sense to create a form.
If you have no CDF defined, you should just be able to place an instance of the pcell (as usual) in the layout and the prompts will be names of the pcell parameters - and all will be fine.
If you want to make the create instance/edit properties form a little cleaner, you may then want to define CDF for the component. This allows you to give nicer prompts to the user. If you do this, please ensure that your CDF default values match the pcell default values (you'll get in a horrible mess otherwise).
To create the CDF, you can start with Tools->CDF->Edit CDF. Make sure you pick "Base" and then add the parameters you need - make sure the types match the types you're using for each CDF parameter.
You may find it easier to define a few, then use:
cdfDump("libName" "fileName.cdf" ?cellName "cellName")
(change the bits in quotes as you need)