The syntax is like this:

http://www.dohop.com/#a1=SXF,TXL,THF&y1=Berlin&c1=Germany&a2=AGP&y2=Malaga&c2=Spain

Where the parameters mean:

a1 = 3 letter IATA airport code(s) for departure
a2 = same for destination airports

Optional information - presets the origin and destination fields with more detailed information:

y1 and y2 = City names
c1 and c2 = Country names

Optional dates - enables dohop to start the search immediately and go direct to results:

d1 = depart date in DDMMYY format
d2 = return date in DDMMYY format (if you skip this one-way search is assumed)