Hi @Heisenberg,
This is unfortunately not possible at this stage. However, other users have requested this before: https://community.hubspot.com/t5/HubSpot-Ideas/Ability-to-set-default-portal-when-logging-in/idi-p/503160
These requests submitted to the HubSpot Ideas section of the community are reviewed by the HubSpot product team, based on their popularity and the assumed demand. I’d recommend commenting and upvoting.
You can also help other HubSpot users find this request more easily (and drive traction) by accepting my reply as a solution. I’d appreciate it, too.
As far as workarounds go, I find there’s a very simple one: log into HubSpot, make sure you’re in the correct portal, and set a browser bookmark. Typically, people either type app.hubspot.com in their browsers of take an even longer path of going through hubspot.com. A bookmark is by far the fastest way to log in. Since it contains the right portal ID, logging in through the bookmark will also take you into the correct portal.
Have a great day!