How can I customize the auto-populated messages that my participants will share?

You can customize the social sharing messages within the Design step of the Campaign Editor within the Share tab.

Based on your campaign's Share URL (configured in the Installation step of the Campaign Editor), when your participants share their unique referral links, the populated image [and some content] will be based on the open graph meta tags on your website. More details here.

Additionally, please note that some social platforms have limitations on what you can customize in auto-populated messages (i.e, For Messenger, you cannot add a pre-populated message).