You have to do the following:
var querystring = require('querystring');
//...
axios.post(authServerUrl + token_access_path,
querystring.stringify({
username: 'abcd', //gave the values directly for testing
password: '1235!',
client_id: 'user-client'
}), {
headers: {
"Content-Type": "application/x-www-form-urlencoded"
}
}).then(function(response) {
console.log(response);
});