Just like you set input’ s name you can access that propertie.

inputs: [{
      name: 'FirstName',
      placeholder: 'First Name'
    },
    {
      name: 'LastName',
      placeholder: 'Last Name'
    }
  ],

  Get them at button handler like

buttons: [{
  text: 'OK',
  handler: data => {
    console.log(JSON.stringify(data)); //to see the object
    console.log(data.FirstName);
    console.log(data.LastName);
  }
}]