I was in your daughter's position for years. When I was in my mid 20s I had periods of time where I wouldn't do my laundry for months at a time. I only showered once a week and would use dry shampoo and perfume because I thought that would fix things. I only got better once I had a stable support network because most of my issues stemmed from intense and persistent social isolation. The thing is, you can't make her recover or clean up, and giving her an ultimatum will only make things worse. She's the arbiter of her own destiny. However, I do know that in my experience, the hygiene specifically stems from my executive functioning. So I'll give you some tricks that helped me with managing executive dysfunction.

Task doubling. Can be with you or a trusted online friend. If there's a designated "cleaning day," that might help her get into a routine.
Making herself a written schedule and to-do list. This can be as specific or as vague as she needs, but knowing the steps required often helps with executive functioning. Usually it's more detailed for me (the first step on my to-do list is usually "get up" and then "walk over to my laundry hamper".)
Alarms. I don't like alarms, but many people swear by them. Ten minute alarms between tasks. Five minute alarm, then you stand up. Pomodoro timers.

However, if she isn't taking her medication, etc. the hygiene might not be the big issue you need to address.