how create clase with atribute asociate to objet server

[mariano]

<field id="Server" xsi:type="select * from server">
      <sql>id_server</sql>
      <default_value/>
      <is_null_allowed>true</is_null_allowed>
    </field>

[Jeffrey Bostoen]

How did you even come up with this xsi:type ?

Can you be as concrete as possible in what you're trying to accomplish?
https://www.itophub.io/wiki/page?id=latest%3Acustomization%3Axml_reference#fields

I think you're looking for AttributeExternalKey