Use expression in combo list to filter out records

I need to show users list from WFUser Table which are in specific Area and for that I have add expression by going to advanced for combo properties but under Basic option Bizagi engine still forcing me to add Data-Source which I did but filter experession doesn't seems to be reacting to this

I am not sure what I am missing here

here is my code

UsersList = CEntityManager.GetEntity("WFUSER").GetEntityList("idArea","5","","");

for(var counter=0; counter<UsersList; counter++)

{

if(counter!=0)

{

if(counter!=(UsersList.Count-1))

{

FilterUsersIDs = FilterUsersIDs + UsersList[counter].SurrogateKeyValue + ",";

}

else

{

FilterUsersIDs = FilterUsersIDs + UsersList[counter].SurrogateKeyValue + ")";

}

}

else if(counter==0)

{

FilterUsersIDs = "(" +FilterUsersIDs + UsersList[counter].SurrogateKeyValue + ",";

}

}

Filterstring="idUser in "+FilterUsersIDs;

Filterstring;