The key to finding out how others do it on their web pages is to look at the "page source," namely the HTML. From there you may need to recover more information.
For example, I went to this page: http://www.youtube.com/watch?v=azkTU_H93sU&feature=mpt%3Atop_stories
Then in my Firefox http client, I did View->Page Source. In all that mess, I wanted to quickly locate in the HTML markup where the URL form control was, so I had a FF add-on that helps web developers by showing me all the ID and CLASS attributes in the web document. I noticed and ID attribute value of "watch-url-field" near the URL textbox control you described. Within the Page Source window, I activated search/find with the text "watch-url-field" and it immediately went to lines 436-440 of YouTube's horribly unvalidated HTML source. These lines read as follows:
I have reformatted them somewhat.
So YouTube web developers created an HTML form which contains only one control, the text box specified by INPUT element. Notice that the INPUT element has EIGHT attributes:
1. name="video_link"
2. id="watch-url-field"
3. type="text"
4. class="email-video-url"
5. value="http://www.youtube.com/watch?v=azkTU_H93sU"
6. onClick="javascript:document.urlForm.video_link.focus();document.urlForm.video_link.select();"
7. onmousedown="yt.analytics.urchinTracker('/Events/VideoWatch/CopyPasteLinkFromMoreInfo');"
8. readonly
The IMPORTANT attribute here is the *value* attribute. This is the URL of the web page, and it was assigned to the value attribute using MOST LIKELY and onload event handler in Javascript, which you can do easily.
Put a SCRIPT element contained in the HEAD element of your web page, and define an onload event handler within it. In order to assign the 'value' attribute of the INPUT element, you will use the standard script function 'document.getElementById() call, with the argument being the value of the 'id' attribute of the INPUT element. In this page, the value is 'watch-url-field' (case sensitive!).
So here is your web page:
=====
This is untested code, but I believe it is correct. Note that the value is assigned the URL, which is Javascript should be the href property of the location object.
Also, the onload attribute is properly ONLY an attribute of the BODY element in the DOM. So after the page loads, the function initPage() is called, and your input element has its value assigned and should appear.
Also note that in strict HTML, an input can be disabled from the user changing it by using the 'disabled' attribute. You can organize the rest of your HTML from there.