We have an external login service and, on login success, we want to redirect the users to our app within Salesforce:
https://our-app-name.sf-server.visual.force.com/apex/our-app-homepage

The problem is that the sf-server is arbitratry.
How to fix this? If we redirect to login.salesforce.com, the user still needs to click his username and then open our app within Salesforce.
We'd like to just redirect to our app.
Thanks!