Adding Cordova plugins to C3 Xcode build

I need help adding any cordova plugins to the c3 xcode build. After trying to add cordova plugin I get this error message:

cordova plugin add cordova-plugin-firebase

(node:15468) UnhandledPromiseRejectionWarning: CordovaError: Current working directory is not a Cordova-based project.

Is there any way to solve this?

Thank you..