To send a request via the sandbox, you can use pm.sendRequest.
pm.test("Status code is 200", function () {
pm.sendRequest('https://postman-echo.com/get', function (err, res) {
pm.expect(err).to.not.be.ok;
pm.expect(res).to.have.property('code', 200);
pm.expect(res).to.have.property('status', 'OK');
});
});