It’s tricky. For security reasons, browsers won’t allow to access their clipboard API, unless the code trying to access it is executed as a result of direct user interaction, if I recall.
For example, pressing on a button will call the backend and it will receive the ExecuteJavascript code you’re trying to use. As it’s asynchronous, that’s not considered direct interaction 
You can create a custom Web SDK button that executes that code directly, on the client side (I have an example if you want it)
Maybe others have more suggestions. 