How to get the current HTTP basic auth headers?

Question

I have a staging site that requires the user to enter HTTP basic auth credentials before accessing the site:

const basicAuth = require('express-basic-auth')
app.use(
  basicAuth({
    users: { fee: 'bar' },
    challenge: true,
  })
)

When I first visit the site I correctly get the prompt asking me to enter the basic auth credentials:

However, after I enter the credentials and try to make a fetch request to the server I need to re-send over the basic auth credentials but how do I get them? I don't want to hardcode them in. Is there a way to access the already used credentials from the browser somehow and send those over instead of replacing them? (Note this JS is inside a webworker):

    const response = await fetch('someURL/', {
      method: 'POST',
      headers: {
        Accept: 'application/json',
        'Content-Type': 'application/json',
        Authorization: 'Basic ' + btoa(username + ':' + password),  <-------------------?
      },
    }).then((response) => response.json())

If I just remove the Authorization header altogether the site won't let me through without the credentials, so the browser is not appending them onto my request.