They have an API: https://platform.deepseek.com/usage; log in with your deepseek account and everything is on the left sidebar (including documentation and creating the key).
I use the API with crush which is an agent, which is made to solve tasks/code, but can also just chat like the web interface. I posted an install guide for it in the sidebar of !crushagent@lemmygrad.ml because it's that easy to set up lol, literally just 7 steps and it should work. With crush you just need to give it your deepseek API key when you first start crush and it should work.
Deepseek's API is also very cheap, I put 5$ in a couple months ago and I haven't finished them yet, and I used literally hundreds of millions of tokens lol. On GPT or Claude this would have lasted me all of 10 minutes.