Do you mean forward all messages that arrive from now on, or do you mean forward all existing messages within a folder?
To forward all messages arriving from now on, you use a rule.
To forward all messages within a folder, it is easiest to use the old classic interface (log in at
https://classic.fastmail.com) select all messages in the folder and then use the option More -> forward as attachments.
If you need more help, clarify which you are trying to do, and we can provide more details.