perhaps the api changed, but i wanted to note this is not correct
/admin/products.json?title=<searchString>,limit=250,page=1,fields="id,title,etc"
rather, you want the extra options added with & and not " so instead
/admin/api/2020-04/products.json?title=<searchString>&limit=250&fields=id,title
also page=1 isn’t supported anymore, see here for updated pagination docs https://shopify.dev/tutorials/make-paginated-requests-to-rest-admin-api