You want to use something like this.

Site url: https://secure.mollie.nl/xml/ideal
Data pair: a|create-link,partnerid|45654623,amount|%cart_total%|description|%image_name%

I was able to make it work, with exception that I don’t have an partnerid, the link will have extra data that is required by Image Store but iDeal payment is processing only the data they require