<label>Are you currently a user of our software?
[select Current-User-Yes-No "Please choose..." "Yes" "No"]</label>
<label style="color:green;font-weight:bold;"> *** Please enter your existing dongle number:[text Dongle-Number]</label>[/group]

<fieldset>
<legend style="padding: 20px 0px 20px 0px;">Package A</legend>
[checkbox* Package-A-Options
"Package A"
"Module 1"
"Module 2"
"Module 3"
"Module 4"
"Module 5"
"Module 6"
"Module 7"
"Module 8"
"Module 9"
"Module 10"
]
</fieldset>

<fieldset>
<legend style="padding: 20px 0px 20px 0px;">Package B</legend>
[checkbox* Package-B-Options
"Package B"
"Module 1"
"Module 2"
"Module 3"
"Module 4"
"Module 5"
"Module 6"
"Module 7"
"Module 8"
"Module 9"
"Module 10"
]
</fieldset>

<fieldset>
<legend style="padding: 20px 0px 20px 0px;">Package C</legend>
[checkbox* Package-C-Options
"Package C"
"Module 1"
"Module 2"
"Module 3"
"Module 4"
"Module 5"
"Module 6"
"Module 7"
"Module 8"
"Module 9"
"Module 10"
]
</fieldset>

<fieldset>
    <input type="checkbox" name="checkbox_1_1">
    <input type="checkbox" name="checkbox_1_2">
    <input type="checkbox" name="checkbox_1_3">
    <input type="checkbox" name="checkbox_1_4">
    <input type="checkbox" name="checkbox_1_5">
</fieldset>

<fieldset>
    <input type="checkbox" name="checkbox_2_1">
    <input type="checkbox" name="checkbox_2_2">
    <input type="checkbox" name="checkbox_2_3">
    <input type="checkbox" name="checkbox_2_4">
    <input type="checkbox" name="checkbox_2_5">
</fieldset>

<fieldset>
    <input type="checkbox" name="checkbox_3_1">
    <input type="checkbox" name="checkbox_3_2">
    <input type="checkbox" name="checkbox_3_3">
    <input type="checkbox" name="checkbox_3_4">
    <input type="checkbox" name="checkbox_3_5">
</fieldset>


<script>
    checkbox_action('1_2', ['2_5','3_1','3_2','3_5'] );

    checkbox_action('1_1', ['2_1','2_2','2_5','3_1'] );

    function checkbox_action(target, relatives) {

        // add a listerner to the target
        const targetCheckbox = document.querySelector('input[name=checkbox_'+target+']');
        targetCheckbox.addEventListener('click', (e) => {

            // when it's clicked, reflect un/checked to it's relatives
            for (i = 0; i != relatives.length; i++)
                document.querySelector('input[name=checkbox_'+relatives[i]+']').checked = e.target.checked;
        });
    }
</script>

add_action('wp_footer',function(){
    ?>
    <script>
        //...
    </script>
    <?php
});