What is the reason for the error: Request header field access-tokens-url is not allowed by Access-Control-Allow-Headers in preflight response?

I keep getting CORS errors saying access-tokens-url isn't allowed, when it's needed.

What am I doing wrong?

The same URL with the same header information works in Postman. Yet I get below errors when I add it to my React UI with axios.

const url = "https://login.esso-uat.server.com

const config = {
  headers: {
    "access-tokens-url": "https://some-access-url",
  }

await axios.get(url, config)
.then((res)=>{console.log(res)}

Error with this code:

Access to XMLHttpRequest at 'https://obfuscated_the_url' from origin 'http://localhost:3000' has been blocked by CORS policy: Request header field access-tokens-url is not allowed by Access-Control-Allow-Headers in preflight response.

Error without the access-tokens-url:

AxiosError {message: 'Request failed with status code 401', name: 'AxiosError', code: 'ERR_BAD_REQUEST', config: {…}, request: XMLHttpRequest, …}