If you are writing an application that doesn't use T1 Node, you'll probably need a proxy of some sort. This package will help you. Create an .env file with at least the value T1APIKEY. Here, you'll put the API key provided by MediaMath developer relations.
To make your life as a developer a little easier, you can also add T1USER and T1PASSWORD. When running your node application in the "development" environment, you can simply point your browser to localhost/api/v2.0/login, and if the .env file has accurate credentials a session cookie will be placed in the browser.
This proxy requires the server running it to use body-parser or another solution that adds POST data on req.body when using the T1 API login route.