Nc_Script_DocID_Get would have to be executed by a script that is in the plugin to get the plugin's DocID (assuming that it is working as documented in the Help file). But it sounds somewhat like you want to call that method from the executed script that is not contained in the plugin.
If that is correct, then you might instead try modifying the plugin such that when it loads, it gets the plugin DocID using Nc_Script_DocID_Get() and generates an .ini file to store that DocID, that gets the plugin's DocID and stores it in the .ini file, then calls the .ini file from the external script to get the plugin DocID. Under this approach, the plugin would overwrite the .ini file anew on each launch of the plugin.
Daniel's Plugin Developer Tools ("PDT"), when it creates a plugin template, embeds a library called Nc_inilazy_hmc that simplifies working with .ini files. If you open the PDT's Help file, inilazy library usage is documented at Tips and Tricks > Configuration / INI file handling: How to use Nc_inilazy_hmc library
I hope this helps.
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