MyCustomApi.php

<?php

/* File: ./custom/clients/base/api/MyCustomApi.php */

class MyCustomApi extends SugarApi {

    public function registerApiRest() {
        return array(
            'myCustomMethod' => array(
                'reqType' => 'GET',
                'path' => array('customEndpoint'), 
                'pathVars' => array(),
                'method' => 'myCustomMethod',
                'shortHelp' => 'This is a custom endpoint.',
                'longHelp' => '',
            )
        );
    }
    
    public function myCustomMethod($api, $args)
    {
      //Some code    
    
      //Here we raise the exception with our custom error message
    	throw new SugarApiExceptionMissingParameter('You are missing a parameter required for properly invoking this endpoint.');
     
      //the rest of your code for your custom method
    }
}

?>