post https://api.usebutton.com/v1/links
Generate and return a link to a Brand identified by their url. This link can then be used by a single user to link to the Brand and make attributed purchases.
Valid values for btn_fallback_exp:
interstitial: Show a branded interstitial to the user to select which fallback they would prefer: appstore or web.appstore: Route user to the platform's app store.web: Route user to the Brand's website.