Its the age old question ha. I think the way things work is
1. Two words
3. One compound word
The more used it is the more it moves toward being a compound work. Like bookstore or fireman. They were once just book and store, and fire and man. But then got hyphenated, and eventually a closed compound.
In this case, it matters not. Since we use the word so much I think we're all justified in using it as a closed compound. But I typically see it the most with hyphen.
English is my 3rd language so I am not an expert in this field.
Both can be used interchangeably in the modern day, but I would say technically the correct use of the word is the hyphenated one.
I use both variants.
When I type down a more sophisticated text, I use the hyphened way to show the readers that I took a considerable time in what I have written, hence took the time to address that the word "add-on" should be written with a hyphen, as it is a made up word.
And when I write one-liners or quick responses (type fast), I ignore it and write without the hyphen. But sometimes I just forget the hyphen, because my mind is bothered with something else.