Declarative Lookup field to external list but no datapicker shown?

Hi,

I am using an own site Feature deploy with it my Content types. I manage to create a lookup field to my external list. Everything works fine. Its getting deployed and is shown as lookup in the site column.

At the document libery I am only able to select the data through a pull down menu (data is correct in the pull down) but I have now a pull down list with 20.000 items. I would like to have the data picker.

When I create the look up column through the create button in site columns it is also working but as a type it is "external data" and I also get the data picker.

What do I miss to declare programmatically?

My working testcode:

  <Field ID="{FCA62BC9-AB0E-428F-96DC-98F1D706F60E}"
     Type="Lookup"
     SourceID="http://schemas.microsoft.com/sharepoint/v3"
     DisplayName="BusinessEntityID"
     Required="TRUE"
     List="6c2d7a82-4f86-445d-afb7-dfc9c5131255"
     ShowField="BusinessEntityID"
     UnlimitedLengthInDocumentLibrary="FALSE"
     Overwrite="TRUE"
     Group="Costume Fields"
     StaticName="BusinessEntityID"
     Name="BusinessEntityID" />