It is Control-Q Control-I. Control-Q is the quoted insert function, and Control-I is the traditional teletype way of typing a tab 0x09 (just like Control-C in a terminal results in 0x03 when not in a cooked mode).

I only found this because I am an Emacs user and I instinctively tried Control-Q first (because many other key bindings like C-f or C-b work). The Control-I bit is a guess that worked. I cannot imagine a non-technical user knowing how to insert a tab character in Safari.