You can code your own model up, but without the model the macro/value won't be calculated so you can't not turn it on and expect Fluent to know the value.
There are macros listed in the documentation, so have a look there. I suspect this isn't what you're after, but may give you a hint https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_udf/flu_udf_ModelSpecificDEFINE.html%23flu_udf_relax_equil
You are navigating away from the AIS Discovery experience