Custom Field and Form Validation

I have created the following custom field

return sprintf(
$html = '<input id="' . $targetName .'-' . $fieldType . '" type="text" name="%s" title="' . $field['field_name'] . '" value=" ' . $meta . '">
        <script>
        document.addEventListener("DOMContentLoaded", function(event) {
            const fp = flatpickr("#' . $targetName .'-' . $fieldType . '", 
            {
                dateFormat: "Y-m-d H:i"
            });
        })
        </script>
        ',
$field['field_name'],
$meta

);

I want to add validation to this and after reading the docs in confused as to how I can make this work with the above hidden input field? basically Flatpickr hides the input field and then uses a html overlay to choose a date. I assume I need to validate e.g. YYYY-MM-DD? or can i just check there value is not null?
https://docs.metabox.io/validation/

[
    'type'      => 'dc_date_picker_calendar',
    'name'      => 'End Date', 
    'id'        => 'end_date',
    'class'     => 'dc-start-date',
    'required'  =>  true,
],

Thanks