GET Request Works in Postman, Fails From Site Page, But Works "manually"

New to APIs/Postman
,
1

My question:
A basic auth GET from my vendor works in Postman, but using the Javascript code generated from Postman fails with a CORs error. But, when I place the endpoint in my browser, and then enter the username/pass in the dialog box that pops up (yes, I know its a bit risky), it works fine.

Why?

I kept the request and response headers from the attempt from my site and the successful attempt from Chrome and can post a sanitized version if that helps.

Details (like screenshots):
Here is the Postman-generated code:

var myHeaders = new Headers();

myHeaders.append("Authorization", "Basic <suppressed>");

myHeaders.append("Cookie", "<Postman is adding a cookie...not sure why but it doesn't seem to be the issue");



var requestOptions = {

  method: 'GET',

  headers: myHeaders,

};



fetch("https://<suppressed>", requestOptions)

  .then(response => response.text())

  .then(result => console.log(result))

  .catch(error => console.log('error', error));

How I found the problem:
console.log and Network/headers

I’ve already tried:
-tried removing the cookie
-tried adding mode: ‘no-cors’ to request. Sometimes that get’s me past the CORS error, but then I get a 401 Unauthorized.
-tried replacing the base64 that Postman provides and perform that encoding in the code but same results.
-tried to refactor code into just a fetch with the second argument being the headers. same result
-tried using the successful request headers from my successful Chrome test. That didn’t work well at all.

2

My opening question wasn’t clear. He’s a second try.

1.A GET from Postman using basic auth works great.
2. It fails when I use the code generated from Postman and place it on the page/site/domain my vendor knows calls will be coming from. I get a cors error
3. It works when I enter the endpoint into a new browser tab and then manually type in the username and pass.

If it works from Postman, and from an open browser window, why wouldn’t it work from the page? More outlined in the Details section. Thanks.