Retain variables when use Guid type

Today, I have problem when create random ID with Guid type in C#.

When I want random ID. And I use:

obj.ID = new Guid(txtID.Text);    //ID type Guid

But when run program. It automatic generate 1 id random when I swipe card.

I want when swipe card. It will random ID and retain this values until I delete it.

Thank you!!!