How can I read the LtpaToken2 token from my XMLHttpRequest response?

Question

I'm trying to debug another developers code which looks like this:

xhr.open("POST", url, true, this.state.userid, this.state.password);
            xhr.withCredentials = true;
            xhr.onload = () => {
                console.log("here is our packet " + JSON.stringify(xhr));
                if (xhr.status === 200) {
                    var test = xhr.getAllResponseHeaders();
                    var respoheader = JSON.stringify(xhr.responseHeaders);
                    var token = respoheader.substring(
                        respoheader.indexOf("LtpaToken2"),
                        respoheader.indexOf(
                            ";",
                            respoheader.indexOf("LtpaToken2")
                        )
                    );
                    console.log("token is parsed ===" + token);

When I run this query from Postman I see two "Set-Cookie" headers added to the response, and one has the content "LtpaToken2=YpMnhu...", which is apparently what I need to grab. But when I run the code above, it does not include this header. I apparently need to grab this token for future calls to another API.

Can someone explain what I'm doing wrong? How can I capture this token, or how am I supposed to connect to another API without this token? As it seems to always be the case with IBM tech, I can find almost ZERO documentation about LtpaTokens.