Good day!
To force HTTPS, you need to use URL rewrite inside IIS. This link from Microsoft should give you everything you need.
https://blogs.technet.microsoft.com/dawiese/2016/06/07/redirect-from-http-to-https-using-the-iis-url-rewrite-module/
Here is another link:
https://blogs.msdn.microsoft.com/kaushal/2013/05/22/http-to-https-redirects-on-iis-7-x-and-higher/