Thanks you very much !

First make sure Thunderbird can use cookies.

• Menu app icon > Preferences
• Select 'Privacy & Security'
• Under 'Web content'
• Select 'Accept cookies from sites'

Then access Account Settings -

• right click on pop mail account name and select 'Settings'

Account Settings will open in a new tab and the pop name of account is auto selected.

• Bottom right click on 'Edit SMTP_server' button
• Set 'Authentication Method' to 'OAuth2'
• Click on 'OK

• Select 'Server Settings'
• Set 'Authentication Method' to 'OAuth2'

Exit Thunderbird and wait a few moments for background processes to complete. Start Thunderbird

You will get prompted by google/gmail to enter your normal password and to allow Thunderbird access. An OAuth token is then stored in the Password Manager in Thunderbird. Then Thunderbird can use token to access server.