This example will include and save padding settings for the input text type fields.

// Add the padding setting in the field.
add_action( 'frm_field_options', 'my_custom_function' );
function my_custom_function( $args ) {
    $field   = $args['field'];

    $is_text_input    = FrmField::is_field_type( $field, 'text' ); // replace 'text' with the type of the field you need to evaluate - for example: radio, checkbox
    if ( $is_text_input ) {
        ?>
        	<p class="frm6 frm_form_field">
				<label><?php esc_html_e( 'Padding', 'formidable' ); ?></label>
				<input type="text" class="frm_repeat_limit" name="field_options[padding_<?php echo absint( $field['id'] ); ?>]" value="<?php echo ( isset( $field['padding'] ) ) ? esc_attr( $field['padding'] ) : ''; ?>"/>
			</p>
		<?php
    }
}

// Save the padding settings
add_filter( 'frm_update_field_options', 'update_field_options', 10, 3 );
function update_field_options( $field_options, $field, $values ) {
    if( $field->type != 'text' ) {
		return $field_options;
    }
        
    $defaults = array(
		'padding' => ''
    );
    
    foreach ( $defaults as $opt => $default ) {
		$field_options[ $opt ] = isset( $values['field_options'][ $opt . '_' . $field->id ] ) ? $values['field_options'][ $opt . '_' . $field->id ] : $default;
    }
        
    return $field_options;
}

In our Public GitHub Repository you can find several examples of what the html of the field settings would be like.

For extra info on how to save a setting once it's added you can follow this example example.