Edit in JSFiddle

$.ajax({
  type:'GET',
  url: 'https://marstree.ltd/api/oauth/token',
  dataType: 'json',
  data: {
    client_id: '5RwTtd',
    client_secret: '2aa8ed89-b5c6-5ad7-ae2a-04ebd27e3c55',
  },
  success: function(resp){
    console.log(resp);
  }
});