By attaching a mailbox, you automatically get the option to use the address of the connected account as the sender's address.
When writing a message, click on the arrow after the sender's address (1.) (the presence of other addresses is indicated by this arrow) and select the desired address from the list (2.).
When replying or forwarding an email, the sender's address is used automatically depending on which mailbox you are in.
Note: Emails sent using the identity of the connected account are stored in the sent folder of the main mailbox.