To quickstart your development you can try out the following client-libraries.
https://www.nuget.org/packages/EconomyApi/
This package simplifies the authentication-process and supplies helper-methods for doing http calls with the Economy REST-Api.
Use Nuget-Package-Manager-Console:
Install-Package EconomyApi
var api = new Economy.Api();
if (await api.Login("bob@test.com", "FancyPass123")) {
var list = await api.GetCompanies();
} else {
Console.WriteLine(api.LastErrorMessage);
}