How Master seed is generated using Bitcore API?
In case you are talking about mnemonic seed use this - 

Install bitcore-mnemonic

npm install bitcore-mnemonic
bower install bitcore-mnemonic


Then use it to generate mnemonic seed - 

var Mnemonic = require('bitcore-mnemonic');
var code = new Mnemonic(Mnemonic.Words.ENGLISH);
code.toString(); 
// army van defense carry jealous true garbage claim echo media make crunch...

var xpriv = code.toHDPrivateKey();


If you just want to generate a private from bitcore api you can use this function - 

var bitcore = require('bitcore');
var HDPrivateKey = bitcore.HDPrivateKey;


Please note there is no rule to generate a private key. A private key is just a random string. It could even be your name or a whole article. But you must create a private key that is 'random' in nature. Also never use programming language functions like md5 hash or something to generate a private key because they are not truly random. 

Bitcore api, however, provides an easy way to generate a private key using the above method. From there you can proceed as usual.

var bitcore = require('bitcore');
var HDPrivateKey = bitcore.HDPrivateKey;

var hdPrivateKey = new HDPrivateKey();
var retrieved = new HDPrivateKey('xpriv...');
var derived = hdPrivateKey.derive("m/0'");
var derivedByNumber = hdPrivateKey.derive(1).derive(2, true);
var derivedByArgument = hdPrivateKey.derive("m/1/2'");
assert(derivedByNumber.xprivkey === derivedByArgument.xprivkey);

var address = derived.privateKey.toAddress();

// obtain HDPublicKey
var hdPublicKey = hdPrivateKey.hdPublicKey;