At this time there isn't a way to customize the Thank You messaging after a contact signs up with the flyout out form. We are collecting feedback for our developers on this, so I will forward your interest along!
If you create a sign-up landing page you would have the ability to customize the thank you page.
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"There's a multitude of ways to engage your audience through us using your social platforms - via ads, social post metrics, email links, and more! " - Will
"Target your most engaged contacts by creating a segment. Create a special offer or show your appreciation!" - Caitlin
"Greet new contacts with one or more automated Welcome Emails depending on their interests or your business goals." - Nick