You're right about chooks enjoying a good soak. My Light Sussex, Harriet had a bumble foot that was as hard as a rock so a soak helped a lot. I was amazed at how much she liked it! It also made my heart glad to see how much cleaner she was as she had had some diarrhea and her bottom feathers were dirty.Give her a good long soak in a tub of warm water or in your sink. This will soften the tissues and make cleaning it up easier. Most chickens love to soak in a warm bath. After she's clean, wrap her in a towel with her head covered to deal with whatever is going on there. It looks like a prolapse, but can't be sure until it's cleaned up to be inspected.
If it IS a prolapsed vent, here are a couple of good articles to get you going:
https://www.backyardchickens.com/threads/successful-vent-prolapse-treatment-with-photos.1101406/
https://vetericyn.com/blog/what-is-prolapse-vent-in-chickens/
(with links to several good articles below)