I've add a new field for the Test Plan which called "reference to" (TS_USER_02). This field is required and drop down box.
And I've add a new field for the Defects(BG_USER_03), the same field as in the Test Plan.
So the value from the required Test Plan field must copy into the Defect field.

Now I use the Workflow script:

Sub Defects_Bug_New
On Error Resume Next
'Befüllt das Reference to Feld
Bug_Fields("BG_USER_03").Value = Test_Fields("TS_USER_02").Value
On Error GoTo 0
End Sub

But the Defect filed (BG_USER_03) does not get the value from TS_USER_02 if I add a new Defect if I use Test Plan -> Run -> Begin Run -> New Defect

Does anybody can help me please to solve this issue?
Thanks a lot!