How to create a base API project and use this endpoint in other API projects?

I have a situation where I have multiple API projects in one solution. What I would like to do now is to create a base project with endpoints, which are available for all the existing API projects.

So, for example:

The base project has endpoint /api/contact

All the other API projects don't have this endpoint. I would like to 'merge' the /api/contact endpoint with all the other projects, so these other project now also have this endpoint.

Off course I can create a new site and host the base project as an independent site, but this get's a new address, I want this endpoint to be available on the other projects, so it keeps the existing host.

I know I need to scan or add the base project somehow to 'add' it to another project, but I have no idea how. I hope you guys can help me out here :)

PS: I am using .net framework; not .net core.