This is how to retrieve headers from any call.
if(Request.Headers.TryGetValue("mandatory-header", out var mandatoryHeader))
{
// may be either filled or empty string
string optionalHeader = Request.Headers["optional-header"];
var result = await _service.DoWork(mandatoryHeader, optionalHeader)
}
else
{
// log error as mandatory-header isn't included in the call
}