Adobe echo sign Authentication api

How can we get the authentication code by calling this api

https://secure.na1.echosign.com/public/oauth?redirect_uri=https://example.com/oauthDemo&

response_type=code&client_id=d4HQNPFIXFD255H&scope=user_login:self+agreement_send:account

as the api redirect the user browser to the adobe site and want the username and password. But we want to implement that feature in our code (c#) by calling the api.

Is there any api in which we can pass username and password also so that we can get authentication code as response in json or other format which will be used in further step to get access token.

Adobe Sign, an Adobe Document Cloud Solution