Sendgrid - Authentication Api Web V3 with User Name and Password - No Api Key C#

I'm working with the web version currently Api V2. Now I need to implement the Web API V3, however, I can not send even an email with my username and password, as it did in each shipping version 2, where by NetworkCredential C #, assigned a user name and password to each shipment.

In version 3 API send only I get Api Key authentication. The problem is that we have many users (username and password) for separating the mailings of different mailing campaigns. And it is very wasteful to create an Api Key for each user.

I want to know how to do to continue using my username and password Api v2 and not have to create an Api Key for each of the users we have. This can be done using JSON ??

I can implement in C # adding user settings and password?

Thank you