Download this browser, open this extension link in this browser and install it. Once extension is installed on kiwi browser:

Go to top right menu, click on "extension" tab, open nos2x extension details, and in extension options you can add your private key. Done

Browser:

https://play.google.com/store/apps/details?id=com.kiwibrowser.browser

Extension:

https://chrome.google.com/webstore/detail/nos2x/kpgefcfmnafjgpblomihpgmejjdanjjp