Symptoms

When navigating to the form runtime URL, the form prompts the user for two authentication methods (see screenshot).

Is there a way to set this to default to STS and not prompt the user? By the way, it looks like this second authentication method was added after connecting the k2 server to SharePoint Online.
 

Diagnoses

I’m afraid this is expected behavior. K2 sites now supports Multi-Authentications and if a different security provider is registered with K2 (i.e. AAD or SQLUM), then you will get the Login prompt method menu.

 

Resolution

http://help.k2.com/onlinehelp/K2smartforms/ICG/4.6.9/default.htm_Introduction_to_Multi-Auth/K2_Multi_Auth_Introduction.htm?TocPath=Considerations and Pre-install Planning|Authentication|Introduction to Multi-Auth|_____0

It might be possible to turn off the login prompt menu but turning off the authentication drop down would have affected the Use for Login option for other users with different security provider, for example AD users under K2 Windows STS Issuer.

You can access the Manage Issuers form (K2Designer > All items > System > Management > Security > Forms > Manage Issuer). Here, you can set the User For Login value to False for Window STS but keep the AAD value to True).