There is no registry value. From the documentation:
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For 64-bit applications (Event Server, Management Client and Smart Client installed as 64-bit):
Executing folder of the starting application:
".\\MIPPlugins".
A common place that all Milestone and OEM applications load from:
[C:\\Program](file:C:/Program) Files\\VideoOS\\MIPPlugins
A common place that all Milestone applications load from:
[C:\\Program](file:C:/Program) Files\\Milestone\\MIPPlugins
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I also recommend: “Debugging techniques for developing XProtect Smart Client plug-ins" and “Debugging techniques for developing XProtect Management Client plug-ins"
https://developer.milestonesys.com/s/article/debugging-techniques-for-Management-Client-plugins
https://developer.milestonesys.com/s/article/debugging-techniques-for-Smart-Client-plugins