Recently had PRP where they spread it on the scalp and used a dermaroller over it, is this the correct method?

Although dermaroller method is useful, it is not equivalent to PRP injections and is not as effective.  PRP injections yield higher concentration of platelet rich plasma and injected deeply into the thick scalp area. From years of experience and research, we use what is called Advanced PRP where we inject PRP directly into the scalp and then follow it with microneedling.
These treatments should be done by professional and experience doctor.

The injection of PRP into the scalp is the ideal method of preventing hair loss. While the dermaroller provides the microneedling effect, it is not equivalent to injections. The scalp skin is thick, making the dermaroller less effective in penetrating it sufficiently for the PRP to work. Good luck!