<?php
add_action('frm_field_options_form', 'my_field_options_form', 10, 3);
function my_field_options_form($field, $display, $values){
  if ( $field['type'] != 'signature' ) {
    return;
  }

    if ( ! isset( $field['label1'] ) ) {
      $field['label1'] = 'Draw It';
    }
?>
<tr>
    <td><label>Signature Options</label></td>
    <td>
    <label for="label1_" class="howto">Draw It Label</label>
    <input type="text" name="field_options[label1_<?php echo esc_attr( $field['id'] ) ?>]" value="<?php echo esc_attr( $field['label1'] ); ?>" class="frm_long_input" id="label1_<?php echo esc_attr( $field['id'] ) ?>"  />
    </td>
</tr>
<?php
}