After installing the plugin, go to Components > RSForm! Pro > Configuration > Notion tab.

Enter your Notion API key here (the Notion Internal Integration Secret). Create an integration to get one at notion.so/profile/integrations, following the steps in Internal integration secret key.

The field is masked, so the secret can't be read off the screen or picked up from a screenshot of this page. To replace it later, paste the new secret over the old value and save.