To link your Larian account to mod.io, first ensure you are logged in to your mod.io account at https://mod.io/g and then simply follow this link and click the "Link your Larian Studios account" button:
https://mod.io/g/baldursgate3?portal=studio
This button will redirect you to the Larian page to login and connect your account.
If successful, you will now find your Larian account listed in your connected accounts here:
https://mod.io/me/account#linked
If you have any questions, please feel free to open a ticket!