API Designer / Server - using http Header Authentication

Trying to implement HTTP Header authentication in the API Server for custom REST Service.

Cannot find the required DLL in include:  "Could not load file or assembly 'HTTPHeaderAuthentifier' or one of its dependencies" on Compile

Any hint or tip on how to get HTTP Header Authentication working in Custom API Server implementation ? 

Enable the HTTP Header Authentication option in Designer - and compiled.

What DLL does this authentication option come in ? According to Designer - the Class is : "VI.DB.Auth.AuthModHttpHeader"

Used VS to browse VI.DB.Auth in VI.DB.dll- no reference to AuthModHttpHeader

Using v 8.1.4