According to the output from aspnet_regsql -? "there is a -d option that you can use, thereby doing your command:
aspnet_regsql -S (local) -E -A -d myDatabase
Alternatively, you can call it with the "-sqlexportonly filenamegoes.here" parameter to create an Sql Script that you can view and then apply to your database:
aspnet_regsql -S (local) -E -A -sqlexportonly output.sql
MSDN SqlMembershipProvider , , , connectionStringName = "SqlServices" .