JSFiddle - React, Tailwind, and code Playground

HTML

<p><input type="radio" name="email" value="[email protected]" />Apple</p>
<p><input type="radio" name="email" value="[email protected]" />Microsoft</p>
<p><input type="radio" name="email" value="[email protected]" />Linux</p>
<p><input type="radio" name="email" value="other" />Other: <input type="text" name="other_email" /></p>
<p><pre id="mailto_link"></pre></p>

JavaScript

$(function() {
    $(':radio[name=email]').on('change', function() {
        if ($(this).val() == 'other') {
            $('#mailto_link').text('mailto:'+$('input[name=other_email]').val()+'?this+is+the+predefined+message');
        }
        else {
            $('#mailto_link').text('mailto:'+$(this).val()+'?this+is+the+predefined+message');
        }
    });
    $('input[name=other_email]').on('keyup', function() {
        if ($(':radio[name=email]:checked').val() == 'other') {
            $('#mailto_link').text('mailto:'+$(this).val()+'?this+is+the+predefined+message');
        }
    });
});