It is not uncommon to stumble upon form code like this that does not have an action or method attribute visible in the HTML code.
<form id="subscription"> ... <button type="submit" >Submit</button> </form>
Surprisingly, when you submit the form, the form gets submitted and you will see a message to that effect. How does this work?
Here is some sample code that shows how this can be done:
# Modifying the form behavior
<script > tag.
So you will have to search the whole source code. Searching multiple files is not a dauting task if you are using a good editor like Visual Studio code or Sublime text.