Hey @PaulCutcliffe , there may to be a few backslashes "" missing from your request, so I wanted to share the body I used:
curl -d '{"smart_collection":{"title":"IPods","rules":[{"column":"title","relation":"starts_with","condition":"iPod"}]}}' \
-X POST "https://your-development-store.myshopify.com/admin/api/2021-10/smart_collections.json" \
-H "X-Shopify-Access-Token: {access_token}" \
-H "Content-Type: application/json"
If you test this but still get errors please feel free to pass on an x-request-id response header returned with the B****ad Request error. I’d be happy to check our logs and see if I can’t pass back any other insights! No rush, I’ll keep an eye out here.