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Hello Yonghui Li
your need for this application, set the IP address on the printer. and in this way achieve the App-printer link.

Like you need now I do not have information that uses an API to achieve this.  (I do not know it, and do not know that it exists to ZPL text) this only exists through own programming such as the example provided.

You can emulate online a by print view page on your web browser:

api.labelary.com/v1/printers/8dpmm/labels/4x6/0/(your code )
p.e. (you code)=^xa  (bla bla bla bla) ^xz

paste your string in address , this line work for label 4*6 inch may be you need other size, You can get information about the topic in.
http://labelary.com/service.html

viewer on line
http://labelary.com/viewer.html

label on line whit the printer

Zebra Technologies - How to Preview a ZPL Label with a Network Printer

Best regards to you and good luck!

// HELLO Yonghui Li, may be you, now has solve, or this solution.

// Printer IP Address and communication port
string ipAddress = "10.3.14.42";
int port = 9100;

// ZPL Command(s)
string ZPLString =
    "^XA" +
    "^FO50,50" +
    "^A0N50,50" +
    "^FDHello, World!^FS" +
    "^XZ";

try
{
    // Open connection
    System.Net.Sockets.TcpClient client = new System.Net.Sockets.TcpClient();
    client.Connect(ipAddress, port);

    // Write ZPL String to connection
    System.IO.StreamWriter writer =
new System.IO.StreamWriter(client.GetStream());
    writer.Write(ZPLString);
    writer.Flush();

    // Close Connection
    writer.Close();
    client.Close();
}
catch (Exception ex)
{
    // Catch Exception
}

// is a sample from the zebra website