selectable-prompt-bugzilla.js

(function() {
    q=window.getSelection().toString().replace(/[^0-9]/g,%20'');
    if (!q) {
        q=prompt('Enter%20Bugid:')
    }
    if (q) {
        window.open('https://bugs.XYZCompany.com/show_bug.cgi?id='+escape(q))
    } else {
        window.open('http://bugs.XYZCompany.com')
    }
})()

You can create a bookmark with the following URI, this when opened, will prompt if there is no selection in the page
javascript:(function(){q=window.getSelection().toString().replace(/[^0-9]/g,%20'');if(!q){q=prompt('Enter%20Bugid:')}if(q){window.open('https://bugs.XYZCompany.com/show_bug.cgi?id='+escape(q))}else{window.open('http://bugs.XYZCompany.com')}})()