It's described in the Deeplink documentation: https://docs.mendix.com/appstore/modules/deep-link/#configuring-constants
and in the SSO documentation: https://docs.mendix.com/appstore/modules/mendix-sso/#using-the-continuation-url-parameter
I recommend you upgrade to Mendix 10 though, and use direct links in microflows instead of the deprecated Deeplink module.