If the Formstack Salesforce Application will not load in Lightning Experience, adjusting your Clickjack Protection settings may do the trick.
From Setup in Salesforce, do a quick search for Session Settings.
Scroll to the Clickjack Protection options.
Uncheck "Enable clickjack protection for customer Visualforce pages with standard headers" and "Enable clickjack protection for customer Visualforce pages with headers disabled".
Once you have unchecked the options, click Save.
You should be able to go back to the Formstack application, and the app should now load in Lightning Experience.