Take a tub of warm (not hot) water outside and set her in it to loosen all the yuck.
You may need to do this several times to get her clean.
Once most of it is loosened or off you can gently bathe her with dawn. Be sure to rinse very well. This will allow you to see if there is anything wrong there too.
after she goes to roost for the night it will be easy for you to grab her and bring her in, I had a similar issue with an EE hen- I trimmed the fluff around her vent and gave her a warm bath and cleaned off all the poop -
so gross! But necessary, she did really well after that, I think trimming her feathers helped keep the mess down. Sometimes poopy butt can indicate laying issues though so keep an eye on her. edited to add- my EE was really wild-not hand raised, but she did not struggle very much, the bath seemed to really relax her. Just keep a firm hold on her wings so she doesn't flap poopy bath water on you before she calms down.. which I had the joy of experiencing when bathing Gertie.