To set the API key in the add-on, navigate to your MultiBaas deployment in a web browser. Login, and go to
Admin > API Keys or
Account > My API Keys.
Create an API key with the appropriate permissions, and copy it to the clipboard.
Back in the Google Sheet, set the API key via the menu
Add-ons > MultiBaas for Google Sheets > Set API Key.
Once you set your API key, for security reasons it will not be retrievable. API keys for the MultiBaas for Google Sheets plugin should be stored in a secure location in case they need to be re-entered, or should be generated as needed.