For forms, builder-hosted forms (Tally, Typeform, Jotform, 123FormBuilder, Fillout, Forms.app, Paperform, among others) are supported if they allow redirect to a thank you page when user clicks submit. For landing pages, you can test any page with a shareable URL: Carrd, Linktree, Link in Bio, Notion, Webflow, Framer, WordPress, and more.
Self-hosted forms are fully supported, just add the tracking link as the submit redirect URL.