Once created copy the Client ID and Client secret. These will be needed when setting up the Web App connection.
When a domain is used on the consent screen or in an OAuth client’s configuration, it must be pre-registered. If your app needs to go through verification, please go to the Google Search Console to check if your domains are authorized. Learn more about the authorized domain limit.
Setting Up Halo
Go to Config > Email > Mailbox Setup > New.
Give the connection a name to identify it within Halo.
Select the Google mailbox type
Add the client id copied from the previous step
Add the client secret copied from the previous step.
Click Sign In With Google to authorise access to the app.
Once completed you should be redirected back to the mailbox setup screen. If the authorisation was successful the Sign In button will have changed to a disconnect button.