Custom created user agent

Hi all Been flicking through the forti docs trying to figure something out. I work with a lot of UC producrs which require remote applications which need access to a lan side pbx. What i want to achieve is creating a inbound rule which allows all but locks it down by a https and custom user agent i can specify based on the application we use. Any thoughrs on how to acheive this. Been looking and i can use the templated agents on the FG but not create a custom user agent string. Let me know cheers