F3+Hin game to enable advanced tooltips, then you will be able to view the item data type after the / like shown below.
64, the number will also need to be a positive number as well.
Knifeas red in the menu.
This is a item, with the color blue with the second line as
description example.with the color blue again in the menu.
[owner]for it to set the skulls owner as the player that is viewing the menu.
SKULL, if you're using 1.13 you will not need a data value and the material will be
(20 ticks = 1 second)and not seconds!
When in main hand:in the menu.
Swordand the value being
0, 1, 2, 3, 4, 5, 6, 7, 8of the menu.
-1to remove it from the menu.