Unlike regular forms, Logic does not recognize hidden input fields, which means that this won't work;
<input type="hidden" name="slug" value="this-page">
Because it's very type-specific, it wants to know the nature and format of what your submitting so that it can be handled internally as a strongly-typed value - an email, plain text, a phone number, and so on.
Workaround
Instead of using an embed with a hidden input type, use a regular input field, and populate it using script. Then hide it using a class to make it invisible to the user.
If you are data-binding content and want to bind the value the way you would in an Embed, you can do this with SA5's Dynamic Attributes and CMS-bind the x-value attribute to have that value automatically set.
- Dynamic Attributes ( SA5 )
Notes
https://discourse.webflow.com/t/logic-trigged-by-form-no-hidden-field/230954/4
