I am trying to figure out how to add a cannon to my PCs' Character sheet.  I looked at EXTRAS, but there is no way to add such.  I would think that it would be a companion, so that I can add it to the maps.  But this is clear as mud and I find no suggestions in the help section.

You back out of the character sheet into the character builder and select which cannon you want there on the Class tab. It is not an Extra.

It is an extra, if I want to create a token to be used in MAPS.  D&D Beyond is so hard to use.  I know that MAPS is in Alpha.  But it just really is hard to trick the system to do the things that you want.

I think there’s been a miscommunication based on a misconception. Officially there are no such things as “extras“ in DND. Extras are not a D&D convention, they are a DDB invention. When I said that the Eldritch Cannon was not an “extra“ what I meant was that they are not implemented in DDB’s system as creatures that can be added under the extras tab of the character sheet, mostly because an Eldritch Cannon isn’t actually a creature. if you want to implement it as a token in Maps, simply Homebrew a monster called Eldritch Cannon, and have your player add it to the extras tab of the character sheet. It will then populate as an available token for you. It’s really quite easy, I do stuff like that all the time. 

Yes, I have to create dozens of "monsters" to create tokens for the map.

Quote from JLeeWatts >>

Yes, I have to create dozens of "monsters" to create tokens for the map.

 
Why dozens? And is the token really all that necessary? Most people I play with have their character hold the cannon in their hand and shoot it like a gun and nobody ever targets the cannon anyway.

I create tokens for monsters and NPCs.  I populate the map with other soft targets and innocents to make the combat more interesting.

I do that too, but if you are creating custom NPCs then you would have to create your own tokens for them. I don’t understand the reason for your complaint?