If your WordPress install and Mastodon server run …

If your WordPress install and Mastodon server run on the same server, wp_http_validate_url() and thereby Share on Mastodon’s settings page may reject your instance URL. (WordPress, somehow, rejects “local” URLs.) The API calls themselves will probably work just fine, though. (wp_remote_post() and its cousins normally do not explicitly “validate” URLs.) This behavior’s “fixed” by explicitly allowing these URLs by means of the http_request_host_is_external filter. https://github.com/janboddez/share-on-mastodon/issues/46#issuecomment-1408315635