Use this code to add a custom display format option to the Radio or Checkbox field.

add_filter( 'frm_radio_display_format_options', 'custom_radio_display_format_options', 10, 2);
function custom_radio_display_format_options( $options ) {
	$options['custom-format'] = array(
		'text' => 'Custom format',
		'svg'  => 'custom-icon-in-option', // You can use filter `frm_images_dropdown_option_image` to change the image or icon later.
	);
}