What is the process for a traveler to Opt-in to receive messaging?

Attached are a few screenshots that should help. The Trip Alert Registration form is what the traveler will need to complete after they click on the "Register Here" link from their Opt-in invitation.

The traveler will then need to complete their Trip Alert profile to setup the "Pre-Trip" and "Traveling" contact points once they've completed the first step. Both Pre-Trip and Traveling contact points are required if a traveler has opted-in to receive messaging. However, if they wish to disable only Pre-Trip alerts, the email address can be updated with a false one (such as example@example.com) to prevent those alerts from being sent to their valid email address.

To resend an Opt-in invitation, please see these FAQ instructions:
https://helpdesk.cirium.com/hc/en-us/articles/217614308-How-can-I-resend-an-Opt-in-invitation-