We do not a have a supported way to edit the favicon on the Formstack-hosted forms. This may be something that we will add support for in the future but in the meantime, we have a few workarounds that will allow you to achieve the desired favicon changes:
1) Advanced HTML
If you have a strong understanding of HTML or have a development team on staff you can find the Advanced HTML Header and Footer editing options by going to the form > Build > Theme. The Advanced Options editor is the option located in the bottom section of the page. We strongly recommend that only those familiar with HTML make these adjustments.
2) 3rd Party Formstack Expert Partners
If you do not have the time or internal resources to make these HTML edits on your own, or are you looking for consultation or help building out your use case(s)? Team up with third party Certified Formstack experts to bring your custom projects to life.
3) Embedding the Form
You can also embed the form on your website by using either the Javascript or iFrame embed code at Publish > Embed Code for the form. With the Form embedded in your own web page, you will have control and of changing or customizing the Favicon through the site hosting the embedded Form.