.NET Email Approval API

I need a lightweight, well-structured .NET (C#) Web API that handles a simple but critical workflow: 1. Accept an incoming JSON payload through a POST endpoint. The schema must remain flexible so I can pass whatever custom fields my process requires without code changes... (Budget: $30 - $250 USD, Jobs: .NET, .NET Core, .NET Core Web API, API Development, API Integration, ASP.NET, Azure, C# Programming)