It finally works. Also I had to use ("x8") not ("X8") when it finally worked.
Also I was initializing as a float, but the docs say it's an INT. Shouldn't make a difference but I changed it anyway. All good now:
Dim mcID As Integer = Device.mcUID
Dim ID As String = mcID.ToString("x8")
Lplan.IFTTT("xxxxxxxxxxxxxxxxxxxxx", "Devlin_Temperature", tempstring, "", ID)