I mean the switch to enable my app to become the default email client app didn't appear on Setting when my app ran on a device. How do I know if it is implemented successfully?

I mean if I successfully implement this functionality, will the switch appear on Setting when the app runs on a device or simulator? Or it will appear only when the app is downloaded from testflight or app store? I followed the instruction to implement it, but the switch to enable my app to become the default email client app didn't appear on Setting when it ran on a device, nor when it was downloaded from testflight. I would like to know if I successfully implement it. If not, how to do it?