Learn how to enable the double opt-in option for custom forms here.
Anyone who submits the form will receive an email to confirm their address. They must click the link in the confirmation email before appearing in your CRM. Select a template that contains the placeholder %recipient.confirm_link% for the confirmation link — otherwise, double opt-in will not work.
To add the confirmation link placeholder:
Go to the Email Marketing section and open the Email Templates tab.
Choose a standard campaign template and click to edit.
Select the text where you want to insert the confirmation link and click the Link tool.
In the opened window, insert %recipient.confirm_link% in the URL field and click Save.
To include the confirmation link in a button, drag and drop a button block onto the template and insert the placeholder link.
Once you have saved your changes, you can select this template as the double opt-in confirmation email for your custom forms.