How can I verify that my users are not robots?

I want to have them enter a code of my choosing to verify that they are not a robot. Please help

What you're describing is called a Captcha Field and you can add one to your form if needed. However, Captchas won't give you the freedom to select a code of your choice to verify your users. The Captcha images are and will always random.

Complete guide: How-to-Add-a-Captcha-Field

There are also other advanced Captcha Widgets that you may want to check out:

https://widgets.jotform.com/widget/google_recaptcha

https://widgets.jotform.com/widget/drawing_captcha

Related guide: How-to-Add-a-Widget-to-your-Form