When and how to request PIP assessment recording

Hello I am in the process of filling out PIP form for first PIP application for bed bound hubby so PIP assessment will need to be over phone or at home. we will be recording the assessment ourselves but we would also like to have it recorded officially .
Please can someone tell me how I go about doing this. I know I need to request this but how and when? Do I have to wait to receive an appointment date before I request it or would you recommend requesting it on the application form? Do I request it over the phone or in writing? sorry to ask.