Patch for domain defined in user properties.

patch.4.0.php

if(!$user = $this->db->get("user",array("api"=>"?"),array("limit"=>1),array($key))) return $this->api_build(array("error"=>1,"msg"=>"A valid API key is required to use this service."));
$this->userid = $user->id;
$this->customDomain = $user->domain;

// Check Request type
if(!isset($_GET["url"]) && !isset($_GET["short"])) return $this->api_build(array("error"=>1,"msg"=>"Please enter a valid URL."));

// Check if shorten request is made
if(isset($_GET["url"])){
       $array = array();
       $array["url"]  = Main::clean($_GET["url"],3,TRUE);
       $array["domain"] = $this->customDomain;
       if(isset($_GET["custom"]) && !empty($_GET["custom"])) $array["custom"] = Main::slug($_GET["custom"]);
       $result = $this->add($array,TRUE);

       return $this->api_build($result,isset($result["short"])?$result["short"]:"");
}

patch.4.1.php

	/**
	 * API
	 * @since 4.0
	 **/
	private function api(){
		// Validate request
		if(!$this->config["api"]) return $this->api_build(array("error"=>1,"msg"=>"API service is disabled."));		
		
		// Check API Key
		if(!isset($_GET["key"]) && !isset($_GET["api"])) return $this->api_build(array("error"=>1,"msg"=>"A valid API key is required to use this service."));
		
		$key = isset($_GET["key"]) ? $_GET["key"] : $_GET["api"];
		// Get user
		if(!$user = $this->db->get("user",array("api"=>"?"),array("limit"=>1),array($key))) return $this->api_build(array("error"=>1,"msg"=>"A valid API key is required to use this service."));
		$this->userid = $user->id;
		$this->customDomain = $user->domain;
		$this->config["captcha"]=0;
		// Check Request type
		if(!isset($_GET["url"]) && !isset($_GET["short"])) return $this->api_build(array("error"=>1,"msg"=>"Please enter a valid URL."));
	
		// Check if shorten request is made
		if(isset($_GET["url"])){
			$array = array();
			$array["url"]	= Main::clean($_GET["url"],3,TRUE);
			$array["domain"] = $this->customDomain;
			if(isset($_GET["custom"]) && !empty($_GET["custom"])) $array["custom"] = Main::slug($_GET["custom"]);
			$result = $this->add($array,TRUE);
			return $this->api_build($result,isset($result["short"])?$result["short"]:"");
		}

		// Check if retrieval request is made
		if(isset($_GET["short"])){
			$g = parse_url($_GET["short"]);
			$g = explode("/",$g["path"]);
			$g = array_reverse($g);
			if(!$url = $this->db->get("url","alias=:q OR custom=:q",array("limit"=>1),array(":q"=>$g[0]))) return $this->api_build(array("error"=>1,"msg"=>"This URL couldn't be found. Please double check it."));
			if((!empty($url->pass) || !$url->public) && $url->userid !== $user->id) return $this->api_build(array("error"=>1,"msg"=>"This URL is private or password-protected."));

			$array = array(
					"error" => 0,
					"long" => $url->url,
					"click" => $url->click,
					"date" => $url->date,
					"location" => json_decode($url->location,TRUE)
				);
			return $this->api_build($array,$url->url);
		}
		return;
	}

lines 3 and 12 added for customDomain registed for the user using API.

Thanks 👍

@kbrmedia added patch for latest version as well, on lines 16 and 25