The fcrepo-message-consumer expects nodes to have a type of "http://fedora.info/definitions/v4/indexing#indexable" and to specify an indexing transformation.  The following steps can be used to set up the namespace and create and register that mixin with your repository.

Define Indexing Namespace and Mixin in CND

External indexing relies upon the objects you wish to have indexed to have an indexing:indexable mixin property. This can be done using the REST interface or via the fedora-node-types.cnd

Definition using the REST interface

Go to http://localhost:8080/rest/fcr:namespaces and in the Register Namespace form add:

Prefix: indexing
Namespace: http://fedora.info/definitions/v4/indexing#

Go to http://localhost:8080/rest/fcr:nodetypes and in the Update CND form add:  

[indexing:indexable] mixin
- indexing:hasIndexingTransformation (STRING) multiple COPY nofulltext noqueryorder

Definition via fedora-node-types.cnd

Make sure your node definitions contain the following:

<indexing = 'http://fedora.info/definitions/v4/indexing#'>

[indexing:indexable] mixin
- indexing:hasIndexingTransformation (STRING) multiple COPY nofulltext noqueryorder

The standard configuration chain is as follows:

1. fcrepo4/fcrepo-webapp/src/main/webapp/WEB-INF/web.xml contains a context-param element with param-name "contextConfigLocation".  The param-value points to your spring configuration file, usually a path like WEB-INF/classes/*.xml
2. One of your spring configuration files is repo.xml that contains a property repositoryConfiguration defining the location of your repository.json
3. fedora-node-types.cnd is defined in repository.json