Connect to CRM as specific user from WebAPI application

I have a question related to create programmatically connection to CRM from external app.

So, I have WebAPI application with Windows Authentication. In the same domain i have on-prem CRM instance.

I want to connect with CRM inside WebAPI application. Unfortunately, I can't obtain password of user, who is connected with WebAPI application. 

1) Does anyone know how to obtain ClientCredentials object in WebAPI Controller method, which is required to create OrganizationServiceProxy?

2) If it is impossible, I have a credentials to admin user in CRM and i can connect in this context. I know that if I have IOrganizationServiceFactory object, I can change user (in plugins for example). Is there any possibility to change service user without IOrganizationServiceFactory object?

Thanks.