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In the S4S Mapping Wizard, map the hidden field in the same way that you would map any of the other fields:
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2. Use custom css class to turn any existing form field into hidden form field.
This method will not require any code compilations or build deployments.
Add the following custom css class to a custom css file which saves in “\sitecore modules\Web\ExperienceForms\css\FormStyles.css” on the file system.
.hidden-field { visibility:hidden; }
Add the CSS file to the Form, or to the Standard Values of the Form template.
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Then add the CSS class to whichever form field you want hidden on the form.
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The form field is visible in the Forms Designer, and within the Mapping Wizard.
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Publish relevant Sitecore items.