Add variable to URL to prefill checkbox

Question

I'm trying to create a link to a page with a signup form and have a checkbox pre-selected, but the variable isn't working.

This is the link:

https://bixocontact.com/apps/Anonym/InterfaceAquisitionPublique/default.aspx?TooteloGuid=eb81e79d-b42e-44f0-bcc3-fbed43a09e4d&BureauId=5327&Lang=EN

Here is the HTML for the form and the checkbox:`

<form method="post" action="./default.aspx?TooteloGuid=eb81e79d-b42e-44f0-bcc3-fbed43a09e4d&amp;BureauId=5327&amp;Lang=EN" id="ctl01" class="ng-pristine ng-invalid ng-invalid-required">

[...]

<input type="checkbox" id="group8842" class="groupInput" ng-checked="group.members.length > 0 || $index == 0" ng-disabled="$index == 0" value="8842">

I tried to adding &group8842=true and &group8842=1 and stuff I already know won't work like /ctl01?group8842=true at the end of the URL to preselect the checkbox for Town of Cardston, but nothing I've tried has worked so far.

I feel like I'm supposed to add the id of the form in the URL variable somewhere, but I'm not sure how to do it on top of all the other variables, and everything I've tried has broken the link. I feel like I'm missing something really simple but I'm struggling to find any answers online.

How can I get that checkbox preselected?