That's right. You don't ADD a Box account ANYWHERE in Thundebird. Visit box.com via a browser and Sign Up for an account, then CONNECT Thunderbird to that account. The button in Thunderbird takes you straight to the login page , so you need to sign up for an account beforehand.

Thanks for the reply. Did that but all the open-new-account options were various paid plans. Nothing about a free Thurderbird-connected FILELINK account. Sent the file via wetransfer.com instead (outside Thunderbirld).

The free plan is under the Individual Plans tab. Direct link is https://www.box.com/en-gb/pricing/individual

https://support.mozilla.org/en-US/kb/filelink-large-attachments https://support.mozilla.org/en-US/kb/how-can-i-send-large-attachment