How do I create an autoresponse to the messages that come through on the contact us form?

How do I create an autoresponse to the messages that come through from the Contact Us form? I read a post that said to download Zapier but does that work for non-gmail accounts? The response would need to come to a business account.  Thank you!

This is not something that you can do inside Squarespace. The solution will depend on the product you are using for your email. The majority of business email solutions (including Gmail which can be customised for business domains) have filters and auto-response options that will enable you to reply to these messages based on the title or content.

My concern is that they'll respond back to Squarespace since they are the ones sending the emails. Do they actually go back to the email accounts that the sender included in the contact form?

@AlisonLee That shouldn't happen if you are prompting your users for an email address in the form.

When Squarespace send you an email that has originated from a form on your site they set the "Reply-To" email address to the email address supplied by the user in the form. You can try it for yourself on a new form.

I just did. When I set up the autoresponder in the ionos mail site, it doesn't seem to auto respond to my test messages. And then I tried auto responding from Apple Mail but that just gives me bounce backs because they're going to "noreply@squarespace".... any ideas?

The reply-to header indicates the email address to use for replies. This is an internet standard. It is different from the From: address (no-reply@squarespace.info). 

From what you say, your email provider is ignoring the reply-to header.

I suggest reaching out to Ionos for support. It's not a Squarespace issue.