JS Varaible To Cold Fusion Variable

i have a select box the user can type a value. this is done using JS.
I now need to take that value and "convert" it over to a cold fusion variable so when the page is submitted, that variable can be used in cold fusion.

I am not sure if this can be done or can be done easily, but converting a js variable to a cold fusion variable in something i really need.

below is the final line of my JS which creates the js variable, and below that is the select box form that i am using.
//what my js looks like after everything has been done
option.text = option.text.substring(0,option.text.length)+ " added";
var alt_string = option.text;

//form field
<select name="NumericAlts#loopcounter#" beginEditing(this);">
<option default=""></option>
<option value="editable">Type New Alt String Here</option>
<OPTION value=some value >some value</option>

<input type="hidden" name="alt_string>

i am using the input type hidden field to try to pass the js value, but it is not working.