Patrick,
If the drop down is an enumeration attribute, you would create a microflow or nanoflow that receives the object from the page, uses enumeration functions to get the server address (getKey or getCaption depending on how you created the enumeration) and then use that value in your microflow or nanoflow.
If the drop down is an association, you would create a microflow or nanoflow that receives the object from the page, retrieves the associated object and then uses the value of the server address found in that object.
Here is an example created using a Nanoflow:
Domain Model
Page
Nanoflow:
Hope that helps,
Mike