$('.wpcf7-form').on('submit', function (e) {
    $.ajax({
        type: "post",
        dataType: "json",
        url: js_object.ajax_url,
        data: {
            action: "custom_reg_code",
        },
        success: function (response) {
            if (response.success) {
                $('#reg_code').val(response.data)
            }
            else {
                console.log('Something wrong')
            }
        },
        error: function (jqXHR, textStatus, errorThrown) {
            console.log('fail: ' + textStatus, errorThrown);
        },
        complete: function() {
        }
    })
})

[dynamichidden custom_reg_code “CF7_custom_reg_code”]

function generateRandomString($length = 10) {
    return substr(str_shuffle(str_repeat($x='0123456789', ceil($length/strlen($x)) )),1,$length);
}

function wpse306816_CF7_custom_reg_code() {

    return date("Ymd") . generateRandomString();
}

add_shortcode('CF7_custom_reg_code', 'wpse306816_CF7_custom_reg_code');