Strange, I just did something like this in a temaplate for the Repository module!
http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/54/threadid/42148/scope/posts/Default.aspx

But I was working on the clientside so had to hardcode it - ok for my purpose.

If you are working on the server you can use this.ModuleID I believe (it's a property of the inherited class I think) - at work so can't check. You'll have to pass this into a RegisterScript method to dump it out in you Jscript. But looking at your question I am not sure you need the module ID - you need the ID of the controls - can you not just hardcode them?

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Every day it seems that I know what I want to do; I just don’t know how to do it.  Getting the client-side ID you mentioned was something I also recently had to tackle.

Here’s what I came up with, although I think the solution revealed above might be better than this (it would be simple and more direct). 
I used a function GetGridID to find my grid.
So for what it's worth here it is:

function GetGridID (psearch)
{
// find a grid by its ID, for example, “id=SMCGrid”
// the id of the grid is cryptic in DotNetNuke
// it will be something like "dnn_ctr366_SMC_central_SMCGrid"
// you must find its "id=" by using this function
// note that it returns to First occurrence (hopefully the one we want!)
var GridID
for (propertyName in document.all)
 {
  if (propertyName.indexOf(psearch)>0)
  {
  GridID=propertyName;
  return GridID;
  }
 }

Then I made use of the above function like so:

function startHighlight ()
{ 
 var GridID = GetGridID("SMCGrid")
}

window.onload = startHighlight;

In the example above I used "GetGridID" to find the ID of a grid a had named SMCGrid.  For the orginal question the code should be altered to be var GridID = GetGridID("my_text_box")

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    - Doug Vogel