To set the deployment ID, navigate to your MultiBaas deployment. Copy the URL from your browser's URL bar.
In your Google Sheet, paste the copied subdomain (MultiBaas deployment ID) through Add-ons > MultiBaas for Google Sheets > Set Deployment ID.
Click on OK.
Once you set your deployment ID, for security reasons it will not be retrievable. Should it need to be re-entered, you will need to re-copy it from the URL of your MultiBaas deployment.