Hello, I new to this program and am trying to add my own clothing for a character. 

So my first issue was a fbxkey? but I followed a tutoral on how to get a key Link Here
and but now once I do that it tells me Failed to load the file Fbx Key is not valid. 
so im not sure how to generate a key for this clothing please help! Thanks 

Follow this procedure instead.
https://manual.reallusion.com/Character_Creator_3/ENU/3/Content/Character_Creator_3/3/08_Cloth/Creating_Custom_Clothes_OBJ.htm
Here is a video tutorial, but start by importing the clothing as accessory:

Fbxkey is only used if you export a character, make changes in an external program, including clothing it, and then bring it back in.

Although it is counter-intuitive, items of clothing created as OBJ should be brought in as Accessory. You can then change the type to Cloth.

The full process is explained here: https://manual.reallusion.com/Character_Creator_3/ENU/3/Content/Character_Creator_3/3/08_Cloth/Creating_Custom_Clothes_OBJ.htm.

EDIT: Rampa was quicker; I had to eat dinner...

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