Can the Value of a Hidden Field Be Overwritten Based on Another Field's Value?

Is this really the only way to get hidden input passed to the email? Can input_3 be conditionally overwritten based on the value of another field? Thanks.

The value of a hidden text box can be overwritten in the same manner as with regular text boxes. The default value in Text Box 2 of this form is replaced with that in Text Box 1 when the latter is filled. The condition which invokes that behavior is shown below:

Text Box 2 can be hidden by clicking Hide Field in its contextual menu:

I'm not sure if this adequately addresses your question though. If it doesn't please elaborate on the functionality you want to achieve.

Thanks