SharePoint 2013 - Consuming external web service

I am trying to create a SSO to connect to an External App in our Extranet platform. I've a custom web service, and with mail address I can get a token. By using a simple console application or ASP web site, I can generate the token, but inside SharePoint, this is not working as expected.

I have a simple WSDL, and by clicking on a link/button, I want to obtain the token. What can I use in order to do that?

I have test webpart (asynchronous is not working), a custom .ashx code calling the web service but without success.

Any help would be appreciate!