You have two options :

1 - Just place the door somewhere you want.

2 - Use a portal / Gateway frame and place the door like a normal gate.

Once you've done that, interact with the door. A text message will appear on the right of your screen saying basically that you need an item called "Mellon" to open the gate.

In the admin pannel, first section (ressources I think ?) type the name of the item.

Now you can use the door.

Note : Once the gate open, the item is used.
You can either make a pile of the item,
Or disable the deletion of the item with the radial menu of the door.