Critical values of r

I've spent several hours trying to track the answer down myself and failed
miserably so a simple question for everyone that will no doubt have a
painfully obvious answer.

What's the command to calculate the critical value of r using a two tailed
test with a given alpha and degrees of freedom? I.e., alpha=0.05, df=18,
two-tailed test, with the result being 0.4438

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