POST with Body Not Passing Cookies
As I commented, I suspect the serialization part. Because when you pass your data as an query string, it works as you expected. So try like this
var qs = require('qs');return axios .post( urlJoin( config.portal.url, 'Account/Register'), qs.stringify({ UserName: "testing_engine@test.com", UserFirstName: "First Name", UserLastName: "Last Name", Email: "testing_engine@test.com", Password: "...", ConfirmPassword: "..." }), { jar: cookieJar, withCredentials: true }) .then(res => callback()) .catch(err => callback(err))