Pair programming session with @Pablo-WMDE and @aniansson led to this result:
https://github.com/wmde/FundraisingFrontend/tree/set-custom-amounts-from-query
Thank you, Pablo!

@kai.nissen There's a difference between the predefined amounts on the donation page depending on skin. On cat17 we have predefined amounts up to 300€, but on 10h16 they're up to 250€. Therefor our current solution fails when the amount given is 300€ and the 10h16 skin is used, because the application considers this a predefined amount but since it's not present in the UI, it's neither selected nor filled in the custom amount form.

We'd like to avoid configuring two different sets of predefined amounts for each skin. We have two proposals:

• The best and least painful solution will be to add a component for 300€ in the old skin as well. Then the input for custom amount will be moved to the right.
• We can add a piece of code that checks if the amount given is predefined but it's 300 so it's considered custom ( along with a giant comment explaining why it's there ) to the js for 10h16 skin.

We're really advocating for the first solution. :)

I'm totally fine with the first solution in general. Not sure, if I understood correctly, but instead of

I propose this

and this