Question Fbx file import problem: How can unity import custom tangent/bitangent vector

Hello to everyone.
I have a problem about unity. I use fbxsdk to write some custom value to tangent vector of a fbx file(data are correctly written to fbx file, I can read it back). But Unity cannot import my custom data. Actually, unity cannot recognize the tangent vector. But the data written to vertex color are imported successfully. How can this happen? If I want Unity to correctly read my custom data in fbx file, what should I do?