K2 can use multiple authentication Issuers. The most common are Forms and Windows Authentication. We can choose which one we would like to auto login with. 

#1. Navigate to K2 Management.

#2. Open Authentication > Claims > Issuers.

#3. You will see a screen similar to this. 

#4. Select which Issuer you would like to use and click Edit. 

#5. Check the Use For Login and click ok.

Now this issuer will be default issuer we use for logging in to Smartforms.