According to the official documentation (https://docs.alfresco.com/process-services1.6/topics/administration_application_config.html):
Configure Alfresco Process Services using a properties file named activiti-app.properties. This file must be placed on the application server’s classpath to be found.
Additionally, the properties file is available with the following options:
An activiti-app.properties file with default values in the WAR file (or exploded WAR folder) under the WEB-INF/classes/META-INF/activiti-app folder.
An activiti-app.properties file with custom values on the classpath. For example, the WEB-INF/classes folder of the WAR, the /lib folder of Tomcat, or other places specific to the web container being used.
The values of a configuration file on the classpath have precedence over the values in the WEB-INF/classes/META-INF/activiti-app/activiti-app.properties file.
I assume if no such file exists you will need to create it in one of those locations.