WordPress Back-End / Integrations Developer — Klaviyo + quiz data wiring

About us Intentive is a CRM and lifecycle marketing consultancy for consumer brands. Our whole business runs on one idea, that every touchpoint a brand puts out either strengthens it or wears it down, and there is no neutral. We are finishing a rebuild of our site, intentive.digital, and we need a strong back-end developer to wire up the data and integrations behind it, cleanly and securely. The project The front end of the new site is built and polished. What we need now is the plumbing behind it: a proper server-side connection to our email platform (Klaviyo) for our contact and newsletter forms, and the wiring that captures results and leads from our new on-site quiz. This is standard-but-important integration work that has to be done securely and reliably, with careful handling of data and secrets. What you will do Build a custom contact form that submits server-side to the Klaviyo API, with the private key held server-side only and never exposed in the browser. On submission it should create or update the person's profile, add them to a dedicated "Website Enquiries" list, subscribe them to marketing only where they have consented, and fire a custom event so we can trigger an auto-reply to them and an internal notification to us. You will migrate our existing newsletter signup to the same clean server-side integration, so everything runs through one reliable path. You will wire the new quiz so that when someone completes it, their result and details are captured into Klaviyo (including their score and result type), working to an integration contract we will provide, since the quiz's scoring engine is a separate build of ours that exposes its data to you rather than something you need to build. Throughout, you will handle personal data and consent carefully and in a GDPR-conscious way, keep credentials secure, and make sure the whole thing fails gracefully and is easy for us to maintain. What we provide Scoped Klaviyo API credentials and the target list IDs. The integration contract for the quiz, defining exactly how the scoring engine hands you its data. The finished front-end pages and forms to connect to. And WordPress admin access to the environment. Must-have skills Strong PHP and WordPress back-end development, including hooks and the REST API. Real experience integrating third-party APIs server-side, ideally including marketing or CRM platforms, with Klaviyo experience a clear plus. Sound understanding of securing API keys and secrets in a WordPress context, and of handling personal data and consent responsibly. Clean, maintainable code and clear documentation of what you have built. Nice to have: direct Klaviyo API experience, familiarity with GDPR-compliant data capture, and experience wiring quizzes, calculators or lead-capture tools to an ESP or CRM. Timeline and budget Paid hourly through Upwork with time tracking, which suits this phase because the exact amount of integration work is still firming up. We will start with a short scoping step so you can give us an informed estimate, then agree a weekly hours cap so spend stays predictable. Figure on roughly a week of work, though the hours will follow the actual scope. How to apply Please open your proposal by telling us, in a sentence or two, how you would keep a Klaviyo private API key secure in a WordPress setup. It is a quick tell that you know this work. Then share an example or two of server-side API integrations you have built, ideally with a marketing or CRM platform, and a line on what you did. Let us know your hourly rate and a rough estimate of the hours, which we can firm up after the scoping step. Start your application with the word 'lemon' so we know you've read this.