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.
Click on OK.
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.