The problem here is with the word "encryption". Icom says that in NXDN mode, it is a digital voice scrambler, with about 32,000 possible keys.
That's not "technically" encryption, but everyone calls it that, so among users and dealers, we just consider it good enough encryption
But yes, it does support that.
In the digital folder on that Encryption page, on line 1 enter whatever code key you want to use.
Then on the Channel page, for that digital channel, turn the Scrambler/ Encryption ON, and the Encryption Key List No. select the code you entered before. Write it to the radio and you are in business.
Now when you are on that channel, you should see an icon that looks like an oval with 2 arrows on the lines (*I think) or something like it.
Of course, on analog channels, you only have the built in option of Voice Inversion, which in this day really does no good for any kind of security, so I wouldnt even bother...
Steven