Well, it seems that we've found a solution, I'm going to resume the full solution.
As the tutorial indicates, to costumize your proxy service endpoint, you have to add the following handler on the axis2.xml configuration:
<handler name="CustomURIBasedDispatcher" class="org.apache.synapse.core.axis2.CustomURIBasedDispatcher"/>
Then, you can customize your endpoint on the design view or on the source view, I've choosen the source view, adding this parameter:
<parameter name="ServiceURI">/services/intern</parameter>
The custom endpoint is autogenerated as:
http://{host}:{port}/services/intern.myWebservice
But I has not worked for me. If I send a request to the custom endpoint, I have no response.
I've solved the problem, adding to the "ServiceURI" parameter the name of the service:
<parameter name="ServiceURI">/services/intern/myWebService</parameter>
then you have to send the request to the following endpoint:
http://{host}:{port}/services/intern/myWebService
So now you have a custom endpoint for every proxy service.
Thanks Malith for your help.