In ASDM go to Remote Access VPN > Network Client Access > Group Policies and select the group policy you would like to change and click edit. In the group policy screen click on More Options, then make uncheck Clientless SSL VPN and SSL VPN Client is checked. Apply the change.
After this change users will hit the SSL VPN web page, log in, and then be connected with the anyconnect client. The credentials from the SSL Login web page will pass through to the AnyConnect client. If AnyConnect was not installed, it will be after the log in.