You can also add folder path to filename in "create document", eg. folder/document.txt
That should solve it if you have only one folder in mind, but if you want to choose it every time, you could generate a list of folders in a current folder at the beginning (I'm sure it's possible but I won't be able to give you a working example quickly).
Edit: Try this: http://www.editorial-workflows.com/workflow/5804267457740800/GF-gZVAMdeU
But it will only work when run from top Dropbox folder.