Every row in the blotter and on the watchlist carries the person who entered it and the person who last changed it. The engine fills those columns; nobody types them, and a client that sends them is ignored. They are ordinary fields, so the Activity page charts them like any other dimension.
Log a trade below and it lands in the blotter with your name on it; double-click a cell in the blotter to change one. A name the desk has not traded before needs a ticker first: the symbol is its id, and it shows up in the pickers at once.