the question you are asking here is already answered in the Q&A/Instruction for filling out the PharmCAS application. If you already have your PharmCAS account, log in and look on the left. It is there in the upper left column as I remember.
the answer to your question is that you do not need to do anything but put your professor's name and contact info in the Reference section on your PharmCAS application. Then PharmCAS will email her with instruction how to write and submit an eLOR online. You do not need to even finish or submit your personal statement for your PharmCAS application as her writing you the LORs or your personal statement does not need to be done in sequence.
Again, for future questions regarding PharmCAS application, I suggest you or anyone applying to thorough read the Q&A/Instruction for filling out the PharmCAS application first. If that still does not answer your questions, call the PharmCAS directly for answers. They are THE ultimate source for ALL questions regarding PharmCAS application.