That should work.
Check the macro settings, you need to make sure you've picked the correct species! But, yes, -1 is mentioned in the text. https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_udf/flu_udf_MultiphaseDEFINE.html%23flu_udf_sec_define_mass_transfer
You are navigating away from the AIS Discovery experience