Indeed, I'm German and of course I know "Heise", but I never heard of their international part "The H!". I would happily pay some bucks regularily to support their journalism, but it seems they don't want my money. (or any other voluntary/donation money for that matter)
They have plenty of paid content you can buy, also gadgets, Arduino stuff, tool sets along with books and magazines in their shop (shop.heise.de). They don't seem to be very good at promoting these things though, probably because they put such an emphasis on their content (it's generally not littered with "buy this" ads).
I don't see how I could explicitly support "The H!" that way. How does Heise know that the customer wants to support "The H!" with that (and not, C't, iX or Heise Online)?
Even if they don't have the right people in their team to run a donation/whatever campaign, they shouldn't have trouble finding those directly among their readers.
Have they tried the "we need money and to rails $50,000 before x or we will shut down" method? That seems to work for many popular sites, where people are happy to contribute $10-$100 for the value in their lives.