Select, Checkboxes, and Radios are all called Option Elements. Each of these has an "other" element, too.

The "other" elements have built-in functionality that allows are user to pick from the options you configure, or write in another value. For example, a user can pick Red, Green, or Blue, or select "Other..." and write in their own value.

You can configure this "other" field, too. You can change the "Other..." to say something else, or change the placeholder prompt, or add a description.