Type ' sqrt($) RET.
If the Q key is broken, you could use ' $^0.5 RET. Or, RPN style, 0.5 ^.
(Actually, ‘$^1:2’, using the fraction one-half as the power, is a closer equivalent, since ‘9^0.5’ yields ‘3.0’ whereas ‘sqrt(9)’ and ‘9^1:2’ yield the exact integer ‘3’.)