Hello @JackDeli,
Welcome to the Klaviyo Community!
I believe something like this can be accomplished in one of two ways; by using either a targeting behavior set on your form or by setting up a custom trigger for the form.
If these contacts originating from your Facebook ads were added to a list or somehow can be captured by a segment, you should be able to target these specific visitors by using the Target visitors in a list or segment under the signup form’s Targeting settings. Both my colleague @Dov and myself have spoke more about using this targeting function in the Community posts I’ve linked below:
If these visitors aren’t being added to a list or cannot be captured through a segment, you can always use the option of a custom trigger. Using a custom trigger will allow you more flexibility as it would offer you the capability to trigger the form based on the behaviors you desire; in this case to only trigger for visitors originating from Facebook. Since this option would require some custom coding to be implemented on your site, we suggest working with a developer you’re familiar with or working with some of our fantastic Klaviyo Partners.
I hope this helps!
David