You can likely do it with your email tool (like Mailchimp) or you can do it with Zapier. It depends on how you have your forms set up.
If you are using an embed from an email tool, then you have to configure sending a confirmation email within the tool. Webflow never actually sees the form submission in this case. Here’s some info on Mail Chimps double opt in (i.e. submission confirmation) About Double Opt-in | Mailchimp.
If you are using Webflow forms, then I would suggest using a Webflow → Gmail Zap. This will allow you to configure a confirmation email once someone submits a Webflow Form.