You can use connection.connection_string_name element in the NHibernate configuration. Have a look herehere. Then NHibernate will get connection string by name from web.config file
You need to use the connection.connection_string_name attribute in the configuration:
<connectionStrings>
<add name="default" connectionString="server=(local);etc." />
</connectionStrings>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
<session-factory>
<property name="connection.connection_string_name">default</property>
</session-factory>
</hibernate-configuration>
With fluent configuration you can do the following
ConnectionString(c=>c.FromConnectionStringWithKey("YourConnStrName"))
With NHibernate configuration API you can do the following:
var cfg = new Configuration();
cfg.DataBaseIntegration(db =>
{
db.ConnectionStringName = "default";
});