For this at first you have to find the external function in services.php of Moodle which u r using,
find out the methodname and classpath, classpath is the path of the methodname.
You have to customize the method parameters.
Example---->
For core_user_create_users external function
'core_user_create_users' => array(
'classname' => 'core_user_external',
'methodname' => 'create_users', #method name
'classpath' => 'user/externallib.php', #path
'description' => 'Create users.',
'type' => 'write',
'capabilities'=> 'moodle/user:create',
),
Have to customize---->
public static function create_users_parameters()
Good Luck