How to import a Facebook Messenger account?
To connect Messenger to Missive, click your name in the left bar, then Accounts and Add account and select any of these two options. You will be first asked to connect a personal Facebook account, then you’ll be able to connect the Facebook Page(s) you manage.
Note that contrary to email accounts, no past history of your Messenger conversations is imported upon connecting those. Only new messages received or sent after connecting the account(s) will appear in Missive. This is not a limitation of Missive but a restriction that Facebook enforces on all third-party apps.