In most universities, selecting a PhD supervisor or guide is a crucial step during the application process or shortly after being accepted. You need to find a faculty member whose research interests align with your own and who is willing to mentor you throughout your PhD journey.
Even if not strictly mandatory during registration, it is highly recommended to start researching potential PhD supervisors well before applying. Reaching out to faculty members whose work interests you demonstrates your initiative and allows you to discuss potential research fit.

https://www.careers360.com/phd-full-form

I hope it helps !