If Statement: want user to be able to enter text in cell if FALSE

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Hi

I have the following formula in cell F16

=IF($M$1 =3, "149028","")

However, what I would really like is for the user to be able to input a
value in F16 is M1 does NOT equal 3.

Is there a way to do this. The current formula leaves F16 blank if M1
does not equal 3, but when the user types another value, the formula is
erased.

what am i missing? Many thanks

.