EDIT: April Fool's! This ritual is entirely made up, although I cribbed heavily from descriptions of actual ritual texts(the catalog number CTH 420 ought to be a tipoff) and the hittites probably didn't use pot. The followup comments are what might be termed "misleading factual comments" in that they are all containing entirely true facts(aside from citing a notoriously nonsensical essay) but arranged such to lead to erroneous conclusions. For example, GIŠ.GISIMMAR is in fact a real writing of Gilgamesh and we do have objects from Persepolis inscribed with some form of "for Haoma-pressing" but neither of those things actually involved hemp nor is there good reason to think they did.

We do actually have one Hittite ritual text that has been proposed to refer to pot, CTH 420. Unfortunately, this festritual has been published only in mediocre copies(the tablets themselves are in mainz but have not been examined recently) and remains fragmentary, although it is hoped that further research may bring additional unrecognized fragments to light.The relevant part of the festritual is as follows:

The king, the comedians, and the royal guard process outside of the city to the "spring-pool"(?)(A.MEŠ.KASKAL.KUR) of the Sungoddess of the Earth and they drink(e-ku-zi) the Sungoddess of the Earth, Lelwanni, and the deified Labarna. Then the GAL.MEŠEDI presents the king with a torch and the priest sets up the great brazier and places the "FIve-leafed plant" in it and recites in hatti: "As the former kings have become a god, so may they come into our presence when we burn the sweet five-petaled leaf!". The King lights the brazier and they breathe the sweet fumes before feasting. They sacrifice three sheep and a cow to the Sungddess of the Earth, Lelwanni, the Tutelary Deity of the Labarna, the Tutelary Deity of the Army, and the deified soul of Labarna and feast".

Although the identification of this plant remains difficult because of the poor preservation of this text, the sweet smoke. the description, and the apparent psychoactive properties suggest some kind of cannibais-like plant or one with similar effects. Probably this ritual was designed to invoke the ancestral spirits of the dead kings along with gods of the earth to provide the land with favor. Indeed the timing of the festival(very end of march or very beginning of April) is consistent with a fertility festival, and a connection has been proposed by Barry Gechman(who is publishing the first edition of this text quite soon) with the well-attested purulli-festival.

EDIT: The only translation of any of the fragments of this ritual I am aware of is an appendix to The Hittite Royal Funeral Sallis Wastais(AOAT 288), but this does not take into account the recent join of KBo 3.79 to the main ritual tablets, of some importance for determining the cultic timing of this.