Checkbox list Question

[Lee NAMBA <le...@atosorigin.com>]

Hello,
This seems like a very typical test use case so I'm puzzled.
I want to recover a list of checkboxes from a Response and send the checked ones in the following response.
Example html:
  <input name="mycheckbox" type="checkbox" value="10" checked>
  <input name="mycheckbox" type="checkbox" value="11" checked>
  <input name="mycheckbox" type="checkbox" value="12">
  <input name="mycheckbox" type="checkbox" value="13">
  
The Request parameters I want to then send with JMeter is:
  ?mycheckbox=10&mycheckbox=11
I am able to extract the values with a regex / for each controller combination:
  Reference Name: mycheckboxData
  Regex:  <input name="mycheckbox" type="checkbox" value="(.+?)" checked>
  Template: $1$
  Match No: -1
  
  ForEachController: My Controller
  Input Variable prefix: mycheckboxData
  Output Variable prefix: mycheckboxDataReturn
  
But then how do you create the multiple request parameters?
  
We've tried constructing a single request parameter with all the parameters concatenated but the User Variables in a ForEachController
does not work.
Any ideas?  Otherwise we're looking into extending the source code.
Thanks