Don't you need to set-up the accounts in the first place? In Thunderbird it's just an ad-hoc text entry field, useful for when you are using a catch-all account

Don't you need to set-up the accounts in the first place? In Thunderbird it's just an ad-hoc text entry field, useful for when you are using a catch-all account and you need to respond from a random service specific email address. For example say my account is dugite-code@example.tld but fetchmail pulls in my gmail emails. If I want to send from house-repairs@example.tld or dugite-code@gmail.com I don't need to configure the three accounts separately I just edit the from field as I'm composing the emails.