How do I make a player fly using the console command line?

In Minecraft 1.13.2 PC JAVA creative mode

What is (is there a) console command to fly in creative mode?

The basic 'in game' option is to double-tap spacebar. I am looking for a console /command line in order to use it within function files.

This question is a split from Ho do i execute multiple function within another function? for disambiguation where i thought the problem was a 'flying' issue. User @Fabian Röling already answered but as OP got more precised, his answer seamed inapropriate.

Asside : i've seen documentation about Effect but can't figure out how to apply on player @s. Here minecraft.gamepedia.com/Status_effect#Levitation

There is no command to make a player fly, but since you can not not fly in Spectator mode, you can just do this:

/gamemode spectator <player>
/gamemode creative <player>

If you want the opposite, you can shortly put someone into Survival or Adventure mode and instantly back into Creative mode:

/gamemode survival <player>
/gamemode creative <player>