Drive adoption
An API in itself doesn't deliver value before it's consumed so it makes sense to spend effort on marketing the internal API to potential internal consumers. The more API consumers an internal API has, the more more value will it deliver and will evolve quicker due to increased funding.
Sharing
The product owner together with representatives of the DevOps team should actively present the current status and plans for the internal API. This can be as a general presentation, but also directly to owners of systems where there may be a potential.
By sharing information on the internal API and lessons learned, you at the same time increase the knowledge of the internal API to the audience and this might be just the thing which creates a new API consumer in the future.
Onboarding process
Make the onboarding process for potential API consumers as simple as possible. Point them directly to the relevant information and follow up the progress or any questions they may have.
If a potential API consumer has a use case where the internal API already has 80% of the functionality, but is missing the last 20%, prioritize the maturing and implementation of the delta functionality high compared to other requests.
Leads
Be active and follow up leads. Be aware of ongoing initiatives and think of if the internal API can deliver value there. Reach out to them informally to discuss the possiblities.
Find new ways of consuming the API
If there is a low-code trend in your, company make sure you treat them as other API consumer and do what you can to make the onboarding process as simple as possible. If a lot of business users are using excel, investigate how you can make the API available there. If a lot of business users are using command lines scripts, investigate how you can make the API available there.
Enterprise architecture
If your company has an enterprise architecture portal, ensure the internal API is properly represented there and points to the API portal for more information.