Display Vendor Contacts as Selector in the PO

Hi, I am new to Acumatica and just did the T200 and T210 Training. 

My customer needs a field that shows all the Vendor Contacts in a Selector (like e.g. when choosing the Vendor in a new PO). 
From here: 

So tried building a field that builds a selector in the POOrderExt of the POOrder DAC. Unfortunately nothing works out for me. I tried it with the following code so far:
 

    #region UsrVendorContacts
    [PXDBInt]
    [PXUIField(DisplayName = "Vendor Contact")]
    [PXSelector(typeof(Search<Contact.contactID,
        Where<Contact.bAccountID, Equal<Current<POOrder.vendorID>>>>),
        SubstituteKey = typeof(Contact.displayName),
        DescriptionField = typeof(Contact.phone1),
        Filterable = true)]
    public virtual int? UsrVendorContacts { get; set; }
    public abstract class usrVendorContacts : PX.Data.BQL.BqlInt.Field<usrVendorContacts> {}
    #endregion

I also checked the datatypes which seem to be int just as my custom field. I also used foreign keys and checked in a GI if there is matching data.

Help would be greatly appreciated!