Let people scan their LNbits wallet QR, which is an URL containing their keys. Use those keys with their standard APIs to allow in-app payments. LNbits also has support for lndhub, so you can use those APIs to connect to the wallet/account as well. One benefit of that is can work as a wrapper over your own non custodial LND, c-lightning etc node or over custodial solutions like lntxbot