If you cannot load the Formstack application in Lightning mode, it is likely that you have Clickjack protection enabled in your org.
These settings can be found in Setup > Security > Session Settings
💡 Clickjack protection is a setting Salesforce enables to prevent pages hosted on external domains from executing in an iFrame embedded in a Salesforce Lightning page. Unfortunately, VisualForce pages hosted within your org are treated as externally hosted. This breaks many native applications that use VisualForce pages (like Formstack). There are several options to fix this issue.
Option 1 (Recommended): Allow *.force.com VisualForce pages
Enable Clickjack protection but allow all VisualForce pages hosted on force.com (Salesforce-owned domain)
Option 2: Allow VisualForce pages on your current instance
Enable Clickjack protection but allow all VisualForce pages hosted on on your specific org.
🔺 Salesforce regularly switches instances that your org is hosted on so the URL may change which means you will need to update the trusted domain when this happens.
Option 3: Disable Clickjack protection on your org
This is the least-restrictive option and is only recommended if the above options do not work.