How can I link my website, app or (online) cash register to the Payconiq by Bancontact app?

To enable your customers to pay with the Payconiq by Bancontact app via your existing payment systems, you don't just have to sign a Merchant Contract. A link needs to be established between your existing payment system and Payconiq. This connection is made by the provider of your system.
Your provider can easily find all details for making the connection via the Website for Developers.
You can find all of the providers who have already made the Payconiq connection here.