The tutorial you are referring to was for Character Creator 1.5 but in Character Creator 3 Pipeline you can bring in custom clothing in a much easier way.
You also shouldn't need to use an FBXKey and instead just choose Create > Accessory to bring in clothing in FBX or OBJ formats. This process is explained in more detail
HERE.
And for a tutorial please click the link below. (Note: You now need to click Create > Accessory and not the way described in the tutorial)
https://youtu.be/nsE5qJP2xAI
Peter
Forum Administrator
www.reallusion.com