Finding similar people is one of the best targeting features Facebook has. If you have your customers emails, you can upload the list to Facebook, and it will find people who are similar to your them.
If you are using Lead Ads, you can use the lookalike capability to target users who are more likely to complete your ad forms.
In the Facebook Ads Manager, go to Audiences, and then click on Create Audience > Custom Audience. Then choose “Lead Form.”
Forms are associated with Facebook Pages, so you need to choose your page first. My recommendation is that you create a Custom Audience with people who opened and submitted your form and then a Lookalike Audience to target similar users.
Prospecting campaigns are always more expensive than retargeting ones. So if you are targeting users who have never been exposed to your brand, you will be better off if those users are similar to the people who already trust you.