How to POST a SOAP message on web api (asp.net core ->web api)

Here is a two-part answer:

Exposing SOAP services with ASP.NET Core:

• Microsoft is not looking into porting SOAP-capable services in ASP.NET Core, not in a near future (look at this other SO answer which states the same).

• That being said, nothing is preventing you from using the plain .NET Framework, which supports SOAP either using old ASMX or WCF which are part of .NET Framework 4.5.x at least (with support ending in 2023).

• Lastly, SOAP seems just a technical requirement for you, so you can build a thin SOAP layer just transfering calls to your main ASP.NET Core service.

The only limitation would be to support deploying this on Linux, on which the full .NET Framework does not natively run on; if not, you're good to go.

Consuming SOAP services win ASP.NET Core:

ASP.NET Core supports consuming SOAP Services in ASP.NET Core through the use of WCF Connected services.