Information about iOS App URL Schemes is here: https://developer.apple.com/documentation/uikit/core_app/allowing_apps_and_websites_to_link_to_your_content/defining_a_custom_url_scheme_for_your_app
Hello,
You can use “milestone” for the currently released iOS client (up to and including version 2019R1) and “com.milestonesys.XProtect” for the future releases (2019 R2 etc)