This recommendation applies to version 5.3.5 of this plugin, but the most recent version is 5.3.6. These findings may no longer be correct.
Findings
Note that this plugin appears to be a wrapper for formget.com rather than doing the work purely within WordPress
This may be a compliance issue for organisations with high data security requirements
It also means that there may be some functionality which cannot be legally tested by third-parties without permission
formget.com is loaded in an iframe over an unencrypted HTTP connection in wp-admin
formget.com assets are loaded over an unencrypted HTTP connection when displayed to the user, at least when using http:// URLs to access the site in question – this appears to switch to HTTPS when the site is accessed via https:// URLs