It’s best to use the category ID integer value as indicated by the docs here:
http://docs.discourse.org/#tag/Topics%2Fpaths%2F~1posts%2Fpost
This is untested by me but I believe you can also use the “subcat” without the prefix “cat/” however there may be some issue if you have categories / subcategories with the same names.
I would stick with integer category ID numbers.