If we look at the help for "Use system proxy", it talks about OPMS version 8.7.
If going to the download page for OPMS, the latest version is 10.7.8. Is this supported on 10.7.8? What's version 8.7 as I have never seen that version on the download page.
Release : SAAS
Component : CA APP SYNTHETIC MONITOR (WATCHMOUSE)
The Use System Proxy property is still supported in OPMS 10.7.8. The help text should state version 8.7 and later. I will have that fixed. In the meantime, you can use that property if you need to with your version of OPMS.