First make sure that IIS is configured correctly. How you can do this can be found in the wiki at this location: https://world.mendix.com/display/Howto/How+to+configure+IIS

Enable the default website and change the setting under the propery HTTP-redirection(this is located in the item Features) and let this redirect to the exact url you want to use with https instead of http. Don't forget to apply the property ‘The exact URL entered above’.

Now select the certificate, select the service and import the certificate you want to use.

Change the 'website' you have created earlier for your application. Change the port that is used to locate the application to some other free port of your choosing, i.e. 8080 or 81. The SSL port must be port 443. These settings can be changed by clicking with your right mouse button on the item SSL settings/Port-bindings. Here you must also select that this website may only be seen through a 'secure channel'. Also select the certificate you have imported in the service and select the encryption type, usually the certificate is a 128 bits certificate.