Hey there zan,
I apologize for the late response, and thank you for being patient! Were you able to fix that on-screen keyboard issue? If not, I'd be happy to help you out. Now, what model of Chromecast do you have? For example, if you have a Chromecast with Google TV, you can use the "Back" button on the Chromecast Voice Remote, which is an arrow.
I look forward to your response, and if you have any other questions or concerns for me, please let me know.