Yes. You sign (instead of login) with your private key (nsec). This can be directly, but its safer to use a signer for this so you don't have to give many tools your private key. One for chrome is nos2x https://chromewebstore.google.com/detail/nos2x/kpgefcfmnafjgpblomihpgmejjdanjjp?pli=1 . Install the extension, go to its settings, paste your key. After this when you go to https://highlighter.com/signup (refresh it if you already have it open), and nos2x extension will show you a sign / authorize popup.