How to run Parse.Cloud.afterSave on registers

I am looking for a way to launch a Parse Cloud application when a user logs into my platform. This assignment will determine his role. Is it possible? I tried with this code but it never works

Parse.Cloud.afterSave(Parse.User, function(request) {
  Parse.Cloud.useMasterKey();
  console.log('launch cloud request');
  if (request.master === false) {
    console.log('not mastered');
    var query = new Parse.Query(Parse.Role);
    query.equalTo('name', 'default');
    query.first({
      success: (default) => {
        var defaultRelation = default.relation('users');
        defaultRelation.add(request.object);
        default.save();
      },
      error: (err) => console.error(err)
    });
  }
});