How to use merge fields in the API?

I'm trying to create a new writer document through the Zoho writer API and I'm wondering how I can fill in the merge fields in my template. I have two merge fields called "Name" and "Address." Here's the code I'm using to create the new document.

writerDoc = Map();

writerDoc.put("Name", "Al");

writerDoc.put("Address", "4738 Street");

document = invokeurl

[

url :"https://zohoapis.com/writer/api/v1/documents?filename=testDoc&template_id=hkne325a8cv1j61r1t1j6m66c6a2fec12293"

type :POST

parameters: writerDoc.toString()

connection: "writerconnection"

];

The problem is that when the new document is created, the name and address fields don't have those two values so does anyone know how I can fill in those merge fields?