Add "select" custom field

Inside the plugin (mu-plugins/alo-easymail_custom-fields.php) you can find this sample that adds a “Favourie music” dropdown to subscription form:

function custom_easymail_set_my_custom_fields ( $fields ) {
	// Custom field: Fovourite music
	$fields['cf_music'] = array(
		'humans_name'		=> __("Favourite music", "alo-easymail"),
		'sql_attr' 			=> "VARCHAR(100) DEFAULT NULL",
		'sql_key' 			=> true,
		'input_type' 		=> "select",
		'input_options' 	=> array(
			"" 			=> '',
			"rock" 		=> __("Rock / Metal", "alo-easymail"),
			"jazz" 		=> __("Jazz", "alo-easymail"),
			"classic" 	=> __("Classic", "alo-easymail"),
			"country" 	=> __("Country / Folk", "alo-easymail"),
			"other" 	=> __("Other", "alo-easymail")
		),
		'input_mandatory' 	=> false,
		'input_validation' 	=> false,
		'input_attr'		=> "style=\"color: #f00\""
	);
	return $fields;
}
add_filter ( 'alo_easymail_newsletter_set_custom_fields', 'custom_easymail_set_my_custom_fields' );