You’ll want mp-destination. From Posting API:
For users who have multiple microblogs configured,
/micropub?q=configwill return of the list of sites. You can post to a specific microblog by passing anmp-destinationparameter of the URL (uidfrom the configured list).
So, that might look something like this in your case: https://micro.blog/micropub/media?q=source&mp-destination=https%3A%2F%2Fsamgrover.micro.blog%2F.