The authorization on the header is not taken into account in my pre-request script

Question

I am trying to establish a pre-request script in postman but I get error "Full authentication is required" because when I send the authorization in the header it is not being taken into account (As I see in the console). This is strange because when I use that same header in a separate request it works fine and generates the desired token.

This is my pre-request code:

var username = pm.environment.get("username");
var password = pm.environment.get("password");

const echoPostRequest = {
   url: myURL(I deleted due the security reasons),
   method: 'POST',
   timeout: 0,
    headers: {
    "Content-Type": "application/x-www-form-urlencoded",
    "Authorization": "Basic xxxxxxxxxxxxx"
   },
   body: {
          mode: 'urlencoded',
          urlencoded: [
            {key: "grant_type", value: "password", disabled: false},
            {key: "username", value: username, disabled: false},
            {key: "password", value: password, disabled: false}
          ]
    }
}

pm.sendRequest(echoPostRequest, function (err, response) {
    console.log(response.json())
    //console.log('Saving the token ')
    //var responseJson = response.json();
   // pm.environment.set('access_token', responseJson.access_token)
});

thank you very much for helping me with this!