Start on your own domain, because that is the only part you control. Publish your OpenAPI at a stable, unauthenticated URL and link to it prominently. Publish an APIs.json index pointing at the OpenAPI, the documentation, the terms, the pricing, the support channel and the status page. Publish an llms.txt if you want machine consumers to find their way around. Make sure your documentation is server-rendered and crawlable rather than locked behind a JavaScript app that a crawler sees as an empty div.
Then go where people already look. Get listed in the catalogs and marketplaces relevant to your space. Publish a collection. Put your definitions in a public repository, because developers search GitHub. Answer questions on the forums where your consumers actually are, because those answers are what search surfaces.
And be findable by problem, not by product name. People search for the thing they are trying to do. Documentation organized around outcomes gets found; documentation organized around your internal service names does not.
One thing worth saying plainly: discovery is increasingly about trust as much as findability. Being listed is not the hard part anymore. Being credible enough — with terms, pricing, status, and a real support channel visible — that somebody chooses you is.
