In the Business Central App you can construct a URL for starting the app on a device, such as a phone or tablet. You can then distribute this URL by e-mail or from a Web page to the users.
The Business Central App URL is based on the ms-businesscentral URI scheme, which is registered automatically when the app is installed. Invoking a URL based on this scheme will start the app with the provided parameters. Below you will find information regarding the URL
So in Business Central Saas, the user would need to open a link similar to this:
ms-businesscentral://businesscentral.dynamics.com/PRODUCTION ENVIRONMENT NAME HERE
For testing purposes lets say your Production Environment is called TEST123 the link would be:
You will have the users open this link on the device via Outlook and this will automatically launch the Business Central App.