How to set required field in contact form 7 wordpress - wordpress

Question

I have a list of multiple checkboxes in the form. The user can check all the checkboxes but at least one is required. Without selecting at least one, checkbox they should not be able to submit the form. how to make it in contact form 7?
I have added the script which is not working.
Here is my script
$(document).ready(function () {
$('#checkBtn').click(function() {
checked = $("input[type=checkbox]:checked").length;
if(!checked) {
alert("You must check at least one checkbox.");
return false;
}
});
});

Answer 1

In this only write star(*) after checkbox element in the plugin form
[checkbox* checkbox "test 1" "test 2"]
[submit "Send"]

Answer 2

It is perfectly work.
On this set your code
$('#fm_submit').submit(function(e){
e.preventDefault();
var ck_box = $('input[type="checkbox"]:checked').length;
// return in firefox or chrome console
// the number of checkbox checked
console.log(ck_box);
if(ck_box < 1){
alert('Select any one box');
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form name = "frmTest[]" id="fm_submit">
<input type="checkbox" value="true" checked="true" >
<input type="checkbox" value="true" checked="true" >
<input type="checkbox" >
<input type="checkbox" >
<input type="submit" id="fm_submit" name="fm_submit" value="Submit">
</form>
<div class="container"></div>

Answer 3

Paste the code from above on the bottom of the form instead of the Additional Settings
$('#fm_submit').submit(function(e){
e.preventDefault();
var ck_box = $('input[type="checkbox"]:checked').length;
// return in firefox or chrome console the number of checkbox checked enter code here
console.log(ck_box);
if(ck_box < 1){ alert('Select any one box'); }
});