The support for Managed schema is enabled by default if you don't specify anything in solr-config.xml.

However, if you wish to make changes, then you can explicitly add your own schemaFactory, as follows:

<schemaFactory class="ManagedIndexSchemaFactory">
    <bool name="mutable">true</bool>
    <str name="managedSchemaResourceName">managed-schema</str>
</schemaFactory>

In the previous code snippet, we have changed managedSchemaResourceName to managed-schema.