Postman

fetch-oauth-token-pre-request-script.js

function getVal(name) {
  if (pm.variables.has(name)) {
    return pm.variables.get(name);
  }
  if (pm.environment.has(name)) {
    return pm.environment.get(name);
  }
  if (pm.globals.has(name)) {
    return pm.globals.get(name);
  }
}

const oauthUrl = getVal('OAUTH_URL');
const clientId = getVal('OAUTH_CLIENT_ID');
const clientSecret = getVal('OAUTH_CLIENT_SECRET');
const scope = getVal('OAUTH_SCOPE');
const resource = getVal('OAUTH_RESOURCE');

const basicAuth = btoa(`${clientId}:${clientSecret}`);

const payload = {
  url: `${oauthUrl}/oauth/token`,
  method: 'POST',
  header: {
    Accept: 'application/json',
    Authorization: `Basic ${basicAuth}`,
    'Content-Type': 'application/x-www-form-urlencoded'
  },
  body: {
    mode: 'urlencoded',
    urlencoded: [{
      key: 'grant_type',
      value: 'client_credentials',
      disabled: false
    }]
  }
};
if (scope) {
  payload.body.urlencoded.push({
    key: 'scope',
    value: scope,
    disabled: false
  });
}
if (resource) {
  payload.body.urlencoded.push({
    key: 'resource',
    value: resource,
    disabled: false
  });
}

pm.sendRequest(payload, function(err, res) {
  pm.expect(res).to.have.status(200);
  pm.expect(res).to.have.jsonBody('access_token');

  console.info('New oAuth token fetched');
  const resObj = res.json();
  pm.variables.set("OAUTH_TOKEN", resObj.access_token);
});