I've hunted down more free romance reads than I'd care to admit, and the Harlequin situation is a weird one. They're a massive publisher, so they don't just dump whole books for free all over the internet. They want you in their ecosystem. Honestly, your best chance is their own website. Harlequin.com has a 'Free Reads' section that rotates titles, usually the first books in a series to hook you. I found the first 'Mira' imprint book there once and ended up buying the next three. It's classic sampler strategy.
Don't sleep on library apps either, that's my real workhorse. Libby or Hoopla, connected to your local library card, will have tons of Harlequin ebooks and audiobooks. The selection varies wildly by library system, but the holds queue tells you what's popular. It's completely legal, supports authors, and you're borrowing not stealing. I've noticed some of the older category romances from the 90s/2000s are easier to get instantly.
Beyond that, it gets sketchy. I've stumbled on sites with uploaded PDFs, but the formatting is always broken, half the chapters are missing, and you're rolling the dice with malware. It's a terrible reading experience and just feels grimy. The official path is less frustrating in the long run, even if you have to wait a few weeks on a library hold. Sometimes the anticipation is part of the fun anyway.