After calling a page with the optin window overlaying the page, the keyboard focus is not in the keyboard focus is not in the optin window, but in the page behind it. The means that you can't enter the optin window without the mouse and you can't enter the cannot enter the page.
The checkboxes in the optin window cannot be operated by keyboard (e.g. by people who cannot use a mouse). However, this is required for barrier-free necessary. Maybe you can make the checkboxes keyboard keyboard - if necessary, you can (optionally) replace them with the standard browser checkboxes. checkboxes of the browser.