If you are developing your own custom script and want to control filtering directly from with AdGuard, request a beta-license key for the app. Note that this version of Assistant is legacy and there is no point in using it adguard userscript new systems, since it has been replaced with.
For problematic sites that change their code frequently, the solution is only included in the AdGuard Extra Userscript version. Still: The Webview does not provide any GM methods. Conceivably, they add their GM API to the currently installed browsers via a browser extension, so it is available there.