Thanks for your replies, I found my answer:
You have to use the SetField and GetField methods. Additionally, you have
to use the FieldNameToFieldConstant method. Take a look at the following
example. Let's suppose your enterprise field name is "E_TASK_Number1" and
you want to set the value of this field for the project summary task. Your
code would look similar to:
Sub Test()
ActiveProject.ProjectSummaryTask.SetField
Application.FieldNameToFieldConstant("E_TASK_Number1"), "480"
End If
You can use the GetField method to read a property. Currently, there isn't
a way to do the same for assignments.