Register a KDE Identity (your username will be combined based on your first and last name given during registration).
After that is done, use the link in the sidebar of this wiki Login with MyKDE. This redirects you to MyKDE, where you need to activate your Identity that was done in the last step.
Back here you are asked if you want to set up a new account or if you want to link to an already existing account, in case you already had one. If so, enter your former username and your old password. If not, simply enter the username you wish to use for this wiki.