I don't see a way to set priorities for MultiBranch projects.
I can see the checkbox for the generated job for a branch `Use specific priority for this Job`, but cannot set it, since this view is Read Only, and no such configuration on parent job.
These appear related;
JENKINS-45030
and
JENKINS-53492
There is now Multi-Branch priority sorter, which extends Priority Sorter but currently suffers from JENKINS-58512.