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.)