Use this code example to add windows-1252 to the end of the API Character Set list of options. It isn't included in the list by default.

add_filter( 'frm_api_charset_options', 'add_api_charset_options' );
function add_api_charset_options( $charset_options ) {
	$charset_options[] = 'windows-1252';
	return $charset_options;
}