PM or email doesn't really matter, but some hosts have deactivated their
PM-function and uses email to communicate with wwoofers. An advice
is to read their presentation carefully and see if they give any
suggestions of what time of year they welcome wwoofers. It is nice
to personalise your letter and mention something about the hosts
presentation as it is some wwoofers who sends out one email to lots
of hosts and then only visits a couple of them in the end. That
can be frustrating when there is a lot to do on the farm and the
internet connection is working slowly.
Some hosts doesn't use their email that much and it could be easier
to reach them with an ordinary letter or make a phone call right away.
Good luck, I hope you will enjoy Sweden!