Go to My Matrix >>Contacts

Click on the name of contact you would like to text. In the contact card, select "Send Opt-In Request"

An email will be sent to the contact's email address, where they can opt-in.

Your client will click on "Sign Up Now," enter their phone number, accept the terms of service and privacy policy, and then select Activate Text Message Notifications.

They will receive the following confirmation.

After they have opted in, you will be able to select "Also send link via SMS text message" when sending emails directly or in the settings of Auto Emails.