Add tag/category via postbody

I have a couple of places on the internet where I use their built-in “post to wordpress” function. In each of these, I can input whatever I want as the body of the post (and the title) but I do not have back-end access for the rest of it.

I would very much like to be able to designate the post’s category or tag when I initially write the post. For what I’m doing I’m always going to use one of 5 tags/categories and most of the time it will be one of two. ( I have no preference between it being a category or a tag)

I figured the easiest way to this would be a shortcode, but I’m just not sure how to go about it. I’ve spent quite a bit of time looking this up today and I found lots of information on shortcodes to display information already existing, or how to alter it on the backend.

Any suggestions? This is driving me crazy.