In short
Social Relay publishes Mastodon statuses on any instance. At connect time the user enters their instance host; Social Relay registers an OAuth app on that instance and completes authorization. Default text limit is about 500 characters; up to four media attachments are supported.
Connect
Include mastodon in platforms[]. The hosted form collects the instance hostname (for example mastodon.social), registers a client on that server, and redirects the user to authorize write:statuses.
Instance differences
Character limits and media rules can vary by instance. Treat 500 as the default validate limit and surface upstream errors clearly when an instance rejects a status.