To turn off the warning message:
Choose Safari > Preferences.
Click Security.
Deselect “Ask before sending a non-secure form to a secure website.”
Sometimes an otherwise secure website may fail to provide a secure way to submit a form, such as a form that contains your password or credit card number. Safari displays a message when this is about to happen so you can cancel. If you don’t want to see this message, you can turn it off.
Choose Safari > Preferences.
Click Security.
Deselect “Ask before sending a non-secure form to a secure website.”