Я наконец нашел ответ на этот вопрос, так что вот решения.
Firsl Создать фильтр WordPress
add_filter( 'gform_confirmation_1', array( $this, 'paynamics_confirmation' ), 10, 3 );
затем создайте функцию paynamics_confirmation
с 3 свойствами $confirmation, $form, $entry
public function paynamics_confirmation( $confirmation, $form, $entry) {
// From Settings
$paynamics_url = $this->get_plugin_setting( 'paynamics_endpoint_url');
$data = $this->paynamics_data($entry, $form, $feed);
$confirmation = '<form name="paynamics_send" id="paynamics_send" method="post" name="paynamics_payment" action="'.$paynamics_url.'">
<input type="hidden" name="paymentrequest" id="paymentrequest" value="'.$data.'">
<div class="paynamics_button_wrapper" style="text-align: center;margin: 0 auto;">
<input type="submit" value="Send To Paynamics" class="paynamics_button" id="paynamics_submit_button" style="background-color: #E46824;border: none;color: white;padding: 15px 32px;text-align: center;text-decoration: none;display: inline-block;font-size: 16px;margin: 4px 2px;cursor: pointer;border-radius: 99px;">
</div>
</form>';
return $confirmation;
}