How to use special value and open box data for enabling next question

QFERT is mixed numeric variable with “open box” and “special value as 98 and 99”.
QCHGENb should be only, if numeric value is 1 to 15 in open box. If opted 98 and 99 by radio button, it should not be enabled.
What syntax will be applied?

QFERT.InRange(1,15)

or what am I missing?