If you want to sell your vote, step 3.

After enabling vote selling, you also need to authorize MinnowBooster to use your account to vote by clicking where asked. Clicking will take you to the authorization page. Click continue and enter your posting key to enable vote selling. Use active key only if you wish to use other services too.

Actually after clicking continue, you end up in v2.steemconnect.com. And here you do need to put your active or owner key. Posting key does not work.
Or am I missing something ?