Probably the easiest is to install a plugin like contact form 7 or similar which sends an email when a form you create is submitted. For all it matters, the fields can all be hidden and only the submit button is visible. You may want to at least provide a quiz or CAPTCHA to reduce unnecessary messages.
You can code your own process by creating a custom page template that presents the form, sending POST submits to itself. When a GET request comes in, present the form. When a POST request comes in, send an email using wp_mail().
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