SetUserAttribute doesn't create it

Hello

I have a user attribute that is synced with Teamcenter. If I try to set it via script it doesn't create it, even tho the documentation says it should:

workPart.SetUserAttribute("MD_DIMENSION", -1, "123", Update.Option.Now)

But if I set it via TeamCenter I'm able to update it via this function.
Am I missing something? How can I set this attribute even if it wasn't set before?