You cannot get a mylfm.eu username/password until navigating to that URL, from your phone, once you land in the EU.
There is no way to assign you a mylfm.eu username or password from their website before you arrive.
Using mylfm.eu is free so you do not need a data pack. However, it's best to add a data pack when you land then navigate to mylfm.eu.
Once you go to mylfm.eu, from your device that has the LFM APN set, and is on the LFM network, you will see a register screen as shown below.
From there, you'll have an account and you'll be able to add more credit, convert credit to data packs and manage your account, from your phone.