Our firm has dealt with this exact issue building out applications for multiple startups/new ventures that have scaled to hundreds of restaurants (including a great company called TableSavvy). There are a few ways you can achieve this without requiring integration into the POS system. it starts first with identifying what platform/back-end system your app is built on (perhaps something custom or something open source like Magento.) Then you have to look at the payment processor you are utilizing and the current method the restaurants are using to apply discounts/codes. It also depends if the restaurants are booking these customers ahead of time or if they are coming in and redeeming real-time/at any time. Based on my experience you have to limit the amount of time any business manager at the establishment needs to put into the effort in order to keep adoption rates high, but there are some tools you can use to reduce their time to a minimal amount. Happy to connect and hear more about your specific needs and craft a process flow/technical recommendation to fulfill it.