[Zax]

OK, I'm now able to /sit
SDPhantom is right: both up and down listener are required for SecureActionButtonTemplate.

EDIT:
As PostClick debugging is now printed twice, I had to add a test before performing PostClick actions.

This is the actual /sit button. I'll test with ground markers this evening and keep you informed.

local frame = CreateFrame("Frame", "Test", UIParent, "SecureHandlerStateTemplate")
    frame:SetSize(40, 40)
    frame:SetPoint("TOPLEFT", UIParent, 10, -150)
     
    local button = CreateFrame("Button", "TestBtn", frame, "SecureActionButtonTemplate")
    button:ClearAllPoints()
    button:SetSize(32, 32)
    button:SetPoint("CENTER", frame)
    local texture = button:CreateTexture("$parent_tex", "BACKGROUND")
    texture:SetAllPoints(true)
    texture:SetTexture(GetSpellTexture(783)) -- "Travel Form"
    button.texture = texture
 
    button:SetMouseClickEnabled(true) -- Make sure OnClick is enabled
    --button:RegisterForClicks("LeftButtonUp", "LeftButtonDown", "RightButtonUp", "RightButtonDown")
    -- button:RegisterForClicks("AnyUp", "AnyDown") -- up and down required for SecureActionButtonTemplate
    button:RegisterForClicks("LeftButtonUp", "LeftButtonDown") -- up and down required for SecureActionButtonTemplate
    
    button:SetAttribute("type", "macro")
    button:SetAttribute("macrotext1", "/sit")
     
    ------ testing purpose
    button:SetScript("PostClick", function(self, btn, down)
        if (down) then print("POSTCLICK: Clicked button=", self:GetName(), "  btn=", btn, "  down=", down, "  macroTxt=", self:GetAttribute("macrotext1")) end
    end)