From https://wordpress.org/support/topic/zip-code-should-be-required-but-is-not?replies=1#post-6858204

Our forms are set up to have "Zip Code" as a required field via the Salsa Dashboard, however when the forms are embedded via the WYSIWYG embed feature, they Zip code is never required, it does not show the * and users can successfully submit the form without entering anything in this field.

The only way I was able to get around this in the past was to use the "text" WordPress editor and manually embed an iframe with the actual Salsa form linked inside it.

Thanks in advance.