The unobvious part was that %C5bo in the query parameters had to be escaped as %25C5bo before this would work. Symptoms in Firefox were that the Javascript became visible in the location bar but nothing happened.
var d=new Date();
location.href='http://www.matkahuolto.info/lippu/sv/connectionsearch?stat=1' +
'&page=criteria&connectionSearchType=departure' +
'&selectedDay=' + d.getDate() +
'&selectedMonth=' + (d.getMonth()+1) +
'&selectedYear=' + d.getFullYear() +
'&departureStopAreaName=Helsingfors&arrivalStopAreaName=%25C5bo' +
'&allSchedules=0&select=20145&ticketTravelType=0&search=search';