Are you asking for the cumulative distribution function (CDF)? See Help > Scripting Index. Then click the button at the top left corner to select Functions. Now select Probability in the first list and F Distribution in the second list. The arguments are the given quantile (F ratio), df for the numerator, and df for the denominator. It returns the probability of this F ratio or less.
I think you are asking for the F Quantile function. (Select it in the second list.) The arguments are the given probability, df for the numerator, and df for the denominator. It returns the quantile (F ratio) corresponding to the given probability.