Well, this rpmb key will be very difficult to obtain, that's why it's called as private key.
With some luck, maybe you can try to collect many engineering firmware as much as possible,
then try to extract it, with rare luck, some might include an rpmb key as a surprise.
We have included several rpmb keys in the RPMB preset menu,
and it is your job to test whether the rpmb key samples we have provided are valid or not.
First make sure the rpmb-key.dat file has been downloaded when you synchronize files to update the software.
After that, you can try the RPMB key preset menu,
choose any key according to your needs,
if you have your own RPMB key,
you can input it in the Enter custom key field...,
after select or enter RPMB key, continue to Special Task, and select the Program RPMB Key/RPMB Provisioning