How to use formmail.php with WordPress

I’m using WordPress as a CMS for a business site. I’ve created an info request form on a Page using formmail.php. Does anyone know which directory I should put the formmail.php file for it to be accessed by the form?

I’ve tried putting it in my theme’s directory and, at a separate time, the root of the site. In both cases, once I’ve filled out and sent the form, instead of taking me to my “thank you” Page, it says, “Sorry, no posts matched your criteria.” I also noticed, the url is “https://mysite.com/request-info/formmail.php” instead of “https://mysite.com/request-info/thank-you” So apparently it’s looking for the formmail.php file under the request-info Page, as if it was a Sub-Page or in that directory, which is why it’s not working. So how can my form access formmail?

Thanks