Grok (on x.com) is nominally constrained in what it can do. It has access to browse_page tool that is able to fetch webpages. It isn't normally able to fill in forms or write messages though.
Since it can browse any arbitrary url, you can set up a site that allows creating messages using only GET requests. This has now been done by myself (holdswarm.com) and a few others.
Proof: https://x.com/civ_enjoy/status/2086081923839918544?s=20