Welcome to www.tftc.duckdns.org. With the help of this website you can log a premium member only (PMO) geocache as a basic member. How...?
Make sure you are logged into geocaching.com with the username you want to log the geocache with.
Then enter the GeoCode, for example GC50F0, in the field below and click on the button "go to logpage". You will then be directed to the corresponding logpage on geocaching.com. There you can write your log.
This also works for logging non-premium geocaches.