After she responds, you throw in a twist – that the dream was about the two of you getting caught in a downpour and getting all muddy.(Or whatever literal use of the word “dirty” you want to go with).
At some point you need to step up and own your own sexual desire for her.
You first text the girl telling her how you had the dirtiest dream about her last night.
If you don’t yet have a strong connection with the girl, you can phrase it like “btw I had the dirtiest dream about you last night” or “omg! ” Mentioning it in passing or adding elements that show you’re somewhat shocked it happened allows you to bring it up in a more casual way.
Another way to help a woman feel comfortable opening up sexually is for you to open up and share your own sexual interests or fantasies first.
By taking the first step you show it’s safe for her to follow you down this road.