There are two ways you can setup Email Subject.
You can add a pre-defined subject by adding a form
type="hidden" along with your subject in
value. See the code below.
<input type="hidden" name="subject" value="New Submission from Web3Forms">
In this case, the subject can be filled by the website visitor. For that, you can use an input
type="text". See Code below.
<input type="text" name="subject" />