You are referring to your documents as "Web Forms". Web Forms are usually embed on your website (or sent as a link) so people can easily access your document to sign without having a sender. It's especially useful if you need to post a form on your own website for signature. Each signer completes the form individually, and each individual transaction is recorded on the Manage tab as a discrete Agreement.. Unless you are using the API, you cannot interact on a Web Form without signing.
Now, if you are using the term web form interchangibly for a regular Adobe Sign send experience, then the easiest way to prefill without signing is to create your document using Prefill field types. When created correctly, these fields will be recognized by Sign as prefill by sender and a sender signature will not be required.