Postman pre-request script to automatically get a bearer token from Auth0 and save it for reuse. Two approaches.

postman-pre-request01.js

const echoPostRequest = {
  url: 'https://<my url>.auth0.com/oauth/token',
  method: 'POST',
  header: 'Content-Type:application/json',
  body: {
    mode: 'application/json',
    raw: JSON.stringify(
        {
        	client_id:'<your client ID>',
        	client_secret:'<your client secret>',
        	audience:'<my audience>',
        	grant_type:'client_credentials'
        })
  }
};

var getToken = true;

if (!pm.environment.get('accessTokenExpiry') || 
    !pm.environment.get('currentAccessToken')) {
    console.log('Token or expiry date are missing')
} else if (pm.environment.get('accessTokenExpiry') <= (new Date()).getTime()) {
    console.log('Token is expired')
} else {
    getToken = false;
    console.log('Token and expiry date are all good');
}

if (getToken === true) {
    pm.sendRequest(echoPostRequest, function (err, res) {
    console.log(err ? err : res.json());
        if (err === null) {
            console.log('Saving the token and expiry date')
            var responseJson = res.json();
            pm.environment.set('currentAccessToken', responseJson.access_token)
    
            var expiryDate = new Date();
            expiryDate.setSeconds(expiryDate.getSeconds() + responseJson.expires_in);
            pm.environment.set('accessTokenExpiry', expiryDate.getTime());
        }
    });
}

postman-pre-request02.js

// Reference: https://nicolaswidart.com/blog/automatically-set-authentication-tokens-in-postman-requests

var authServiceUrl = pm.environment.get('authService');
var gatewayBaseUrl = pm.environment.get('gatewayBaseUrl');
var username = pm.environment.get('username');
var password = pm.environment.get('password');

var sdk = require('postman-collection');

var isValidTokenRequest = new sdk.Request({
    url: gatewayBaseUrl + "/api/item/items", // Use an endpoint that requires being authenticated
    method: 'GET',
    header: [
        new sdk.Header({
            key: 'content-type',
            value: 'application/json',
        }),
        new sdk.Header({
            key: 'acccept',
            value: 'application/json',
        }),
        new sdk.Header({
            key: 'Authorization',
            value: 'Bearer ' + pm.globals.get("jwttoken"),
        }),
    ]
});

pm.sendRequest(isValidTokenRequest, function (err, response) {
    if (response.code === 401) {
        refreshToken();
    }
});

function refreshToken() {
    var tokenRequest = new sdk.Request({
    url: authServiceUrl,
    method: 'POST',
    header: [
        new sdk.Header({
            key: 'content-type',
            value: 'application/json'
        }),
        new sdk.Header({
            key: 'acccept',
            value: 'application/json'
        }),
    ],
    body: {
        mode: 'raw',
        raw: JSON.stringify({
            username: username,
            password: password
        })
    } 
  });

  pm.sendRequest(tokenRequest, function (err, response) {
      if (err) {
          throw err;
      }
      
      if (response.code !== 200) {
          throw new Error('Could not log in.');
      }
      
      pm.globals.set("jwttoken", response.json().token);
      console.log(`New token has been set: ${response.json().token}`);
  });
}