As we’re not experts in 3rd party gateways like this, I’d advise you to reach out to contact Mollie directly to ask them about how you can set this up for your store.
They should be able to provide advice and support for how you would activate this specific payment method through them.
If they provide you with any instructions or steps on how to put this in place in your Shopify admin, let us know here so we can help you put them in place!
Are you still getting set up with this store, or have you been selling for a while already?