Effective today, you can log into the forum by using your KDE Identity (identity.kde.org) credentials. This feature is still under development, so for now, you'll need to link your Identity account with your forum account over here: ucp.php?i=profile&mode=reg_details (UCP -> Profile -> Account Settings).

Once that is done, you can select the "Log in using KDE Identity" option on the login screen to use your KDE Identity credentials. Please note that one Identity username can be linked with only one forum account.

Update: Plugin is now complete. You can directly use your Identity credentials at the login screen. In case it is not linked, the plugin should let you link it to an account or create a new user.