Configuration form confirmation I have a configuration form which extends ConfigFormBase.

How can I add a confirmation popup/page to give a message to the user that some field has certain value and do you wish to continue form submission?

I know there also is the ConfirmFormBase class but I don't have a clue how to implement this in my configuration form.
I'm not sure you could easily implement ConfirmFormBase with the config form. It might be easier to turn your form into a multistep form and have step 2 with a simple submit button/message for confirmation rather than trying to implement ConfirmFormBase 

That should achieve essentially the same thing