Need help changing value in textbox?

Hi, I currently have this code below which when I change the value in the dropdown list (id=changer) the value in the textbox also changes, try it out you'll see what I mean. As you can see "TEXTHERE" appears each time. Now say I need to use this script more than once on the same page, but with a different word instead of "TEXTHERE" each time. How would I go about doing this without having to write a seperate script each time? Sorry if you don't understand, I found it quite hard to explain. Would really appreciate any help, Thank you

<script type="text/javascript">
function change( txtBox )
{
var changer = document.getElementById('changer');
if(changer.value != "1"){document.getElementById('ref').value = 'TEXTHERE{$encode}';}else{document.getElementById('ref').value = 'TEXTHERE{$signin_username}';}
}
</script>

<select id='changer' onclick="change(this);" >
<option value="1">{$signin_username}</option>
<option value="2">{$encode}</option>
</select>

<input type="text" value="TEXTHERE{$signin_username}" readonly="readonly" id="ref" onclick="document.getElementById('ref').select();" />

HTML:

 

<script type="text/javascript">
function change( txtBox, resp_str )
{
var changer = document.getElementById('changer');
if(changer.value != "1"){document.getElementById('ref').value = resp_str + '{$encode}';}else{document.getElementById('ref').value = resp_str + '{$signin_username}';}
}
</script>

<select id='changer' onclick="change(this, 'lol wut');" >
<option value="1">{$signin_username}</option>
<option value="2">{$encode}</option>
</select>

<input type="text" value="TEXTHERE{$signin_username}" readonly="readonly" id="ref" onclick="document.getElementById('ref').select();" />

HTML:

this script will do the same thing, except that it will show "lol wut" instead of "TEXTHERE".
the line

onclick="change(this, 'lol wut');"

Code (markup):

is the one where you can set the string to a different value

 

exactly said. This(not the this in the code) will change the current content to your desired text.