Earlier quoted context omitted.
I always worry with plane tickets because I have a middle name on my passport but many buying services don't take middle names into account. Also the name of my town has a "Ą" letter in it, which also is problematic in online forms and I often just write A instead just to be on the safe side.
By definition there is no legal middle names here in Finland (all of the 1 to 4 names given to you are called "first name" in the law). And thus all of your names will be in the passport too. Filling some foreign (or poorly done/ported local) it system forms can be bit of a guess work. Also as a bonus space " " is a legal character in a name. Both "Jukka Pekka" and "Jukka-Pekka" are valid names (also you could have 2…
For example, in Latvia, the legal treatment is that if you have two first names (two is the legal limit) then they are space delimited and if you have two surnames (it's becoming popular to join the surnames after marriage instead of changing the surname of one spouse) then they are hyphenated.
So if you see "Alpha Beta Gamma" then that means Alpha and Beta as given names and Gamma as the surname; and "Alpha Beta-Gamma" means that Alpha is the given name and Beta-Gamma is the surname, so the name in any official documents can be unambiguously parsed.