I would guess that the delegated user would have to add the delegated account into Thunderbird as if it were a new account. As far as I can see, it just adds an additional user to an existing email account, albeit with some restrictions.

I would presume that the delegate would have full access to all your folders and hence your back catalogue of messages.

If you want to add a delegated account if it were a new one, you need the password of the user. This password will change every 50 days.

Also it shouldn't be needed to use a password because it is delegated