Sorry! I wasn't very clear on my question. Which I why I don't write articles.
My question is: Can I send this article to an Upwork writer and say "I want an article on this subject for my newsletter". Is that enough or would they need more info?
That's probably enough. They'll likely ask any questions they have to clarify, such as the tone of voice you want. Ex: are you OK with humor occasionally, or do you want it to be more serious? That type of thing. Also, make sure you tell them your audience if it's not already clear. Who will end up reading this? That always helps.