Thanks for the hint 
So the dirty (but save) workaround looks like this in Coresuite API translation:
' Open new CustomerOrder Form in ADD mode:
| SwissAddonFramework.UI.Components.Form.OpenFormByKey("17", Nothing) | |
Dim oForm As SwissAddonFramework.UI.Components.Form = SwissAddonFramework.UI.Components.Form.GetFormFromType("139", 1) oForm = SwissAddonFramework.UI.Components.Form.GetFormFromType("139", oForm.TypeCount) | |
| oForm.Mode = SwissAddonFramework.UI.Components.ModeComponent.FormModes.ADD |