Functionality: I want to make my trinket (on-use item that has been dragged to my action bar) to 'flash' the same way a spell would when a buff was active.

115154 is the action data id AND item data id for the trinket on the actionbar, obtained with SpellDevInfo

return PassiveModifier {
157217, -- Shadow Orbs
115154, -- On-use Trinket
126705, -- Passive Trinket (could be any buff though, heroism for example)
"player",
"flash"
}

When I change the item (115154) to a spell (10060 - power infusion) this code works fine. Is there a solution?

Yep, for items you should use "item:115154" instead of 115154 (I think, I haven't tested it).

Works great. You can close this. Thanks again

Copied this rule to https://github.com/Adirelle/AdiButtonAuras/wiki/Example-rules.

Here's another 'trinket' example should someone need help making a trinket rule.

return Configure {
            "Pol's Blinded Eye",
            L["Flashes when Pol's Blinded Eye is off cooldown."],
            "item:113834",
            "player",
            "UNIT_AURA",
            function(_, model)
                if not GetPlayerBuff("player", 176876) then
                    model.highlight = "flash"
                end
            end,
        }