Default set CustomField value

Hi, Does someone know how to do this:

  1. create a CustomField (such as CFU) for users;
  2. input its value (such as CFU_A) when create a user
    A;
  3. create a CustomField (such as CFT) for tickets.
  4. when user A create a ticket, how to default set
    CFT’s value as CFU_A?

Thanks for your haelp, Zhiming.

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