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