Create a form to get user information for payment gateway

I found the answer, and I put here for anyone who want to this:

For showing custom fields below codes should add to function payment_fields:

public function payment_fields(){
    if ( $description = $this->get_description() ) {
        echo wpautop( wptexturize( $description ) );
    }
    ?>
    <div id="custom_input">
        <p class="form-row form-row-wide">
            <label for="TedadGhest" class="">
        <?php _e('number of installment', 'oti_iran_hekmat'); ?>
    </label>
            <select name="TedadGhest" id="TedadGhest">
      <option value="6">6</option>
      <option value="12">12</option>
      <option value="18">18</option>
      <option value="24">24</option>
    </select>
        </p>
</div>
    <?php
}