C# api?

Is there/ is it possible to send data to piwik from a c# application or something like that?

yes

See libs/PiwikTracker for the PHP tracker (and soon, libs/PiwikTracker/java).

Any documentation or something on how to use it?

try c# socket for coonection How to C# Socket programming

wills.

C# API Clients are now available : C# Tracking and Analytics API Clients Available Now - Analytics Platform - Matomo