Welcome to the Workshop community and many thanks for making your first post. So that our community members might be able to provide more helpful advice, could you be a little more specific about what problem you are experiencing with your pressure washer in regards to the detergent function? I take it you are not finding the information in the product manual helpful?
Sorry to read of your troubles. You might find the advice in this previous discussion helpful - https://www.workshop.com.au/t5/Outdoor/Ryobi-2000PSI-pressure-washer/m-p/9822
Let me extend a very warm welcome to the Workshop community. We look forward to reading about your projects and plans and trust our members will be able to provide loads of helpful advice and inspiration along the way. We have clever and creative members sharing amazing projects every day.