Thanks Bernard for your prompt response. I will check that further, but I noticed that the existing Tracker plugin, when filled and sent, does update the existing record and does not add a new one. I.e. when opening the page, the Name field appears empty (even though the connected user already has one registered in the tracker) and if the empty field is completed and the form sent, the name in the Users tracker is updated (the name is replaced with the new value for the corresponding user). Therefore, I am surprised that I would need to specify the ItemID (obviously, Tiki already knows it).
But I will check your suggestion.