I've decided to enhance the mechanism os composing keys. With the original Ducky Scripy it is only possible to compose keys with SHIFT, ALT, CONTROL and GUI.
The solution to allow several keys to be pushed is use a flag Hold_Next_Key which simply tells the Ugly interpreter to not send a key release after the next valid key.
With that, the combination keys tokens 0xa8 to 0xab have the same effect that is to set the Hold_Next_Key flag.

With that in mind a GUI R command shall be sent as:

hold  gui    r
0xAB  0xE3  0x15

Then a CONTROL ALT DEL should be sent as:

hold  contol  hold   alt   del
0xAB   0xE0   0xAB   0xE2  0x2A