Rich Text Editor for ASP.NET Core is available as a CTP (community technology preview) in version 18.2 and doesn't provide all customization features. At the moment, it doesn't provide the capability to add a custom toolbar item and modify the document's text on the client side. Our developers are aware of this limitation and have this functionality in our plans for further versions. Currently, I cannot provide you with any ETA on when it will be available. Follow our updates to be notified of our progress with this.
Let us know if you need further clarification.