Lookup on filed based on selected BaseEnum element in another field.

Create new class implements CaseIAssociation. 
Write attribute as shown in snippet.  
[ExportMetadataAttribute('CaseIAssociation', 'BaseEnum ElementName'),ExportAttribute('Dynamics.AX.Application.CaseIAssociation')] 
class ProjCaseAssociationProvider implements CaseIAssociation


In method performLookup() write your lookup logic. 
if (_caseAssociation.EntityType)
        {
            callerControl.performTypeLookup((new SysDictField(
                    tableNum(ProjTable),
                    fieldNum(ProjTable, ProjId)).typeId()));
        }
​ 
Eg:Class ProjCaseAssociationProvide