Any POD is fine. How can Imperial Russia be made to have ownership over the entirety of Hokkaido, either through settlement or some form of conquest?
I don’t see how this could be done short of the early major changes in the Russian foreign policy, economy, demography, etc. In OTL even claim to the whole Sakhalin was not backed up by an adequate number of the settlers (including prisoners) and Southern Sakhalin remained almost empty. Not that there were any serious trade interests on the Pacific except those related to China and even those were predominantly inland.