Multiselect Checkbox not rendering/submitting as array


I have a form with Multiselect Checkboxes, and when I embed the form on my page it renders each <input with the same name but no square brackets. So when the form is submitted, only one key comes through.

Here is an example of how it’s rendering:
<input type=“checkbox” name=“my_multichk” value=“val1” />
<input type=“checkbox” name=“my_multichk” value=“val2” />
<input type=“checkbox” name=“my_multichk” value=“val3” />

Here is how it SHOULD be rendering so all checked values make it to POST:
<input type=“checkbox” name=“my_multichk[]” value=“val1” />
<input type=“checkbox” name=“my_multichk[]” value=“val2” />
<input type=“checkbox” name=“my_multichk[]” value=“val3” />

Any idea how to make it render as a checkbox array?