On Saturday I learnt that eBikes do actually have on/off switches for the lights. Some of them, anyway, but like BMW indicators they're rare in this neck of the woods.
Lots of ways that can work, depending on the system and lights involved, but why did you think they didn't?
Of course, with an ebike, the penalty for having your lights on is practically non-existent. It's a vanishingly small amount of energy compared to the total capacity of the battery (which like any motor vehicle user, you'll have planned your journey around not running flat). You're already going to recharge the battery, and it doesn't even create extra drag like a dynamo hub. So if you're inclined towards thinking that daylight running lights are a good idea, there's no reason not to.
Data point: With Shimano STEPS (which is a newish high-end system, and relatively rare to spot in the wild, but it's one I happen to have read the manual for properly) the lights are controlled by a prominent button on the console, and optionally tied to the display backlight state. This seems typical of well-integrated off the shelf ebikes. A minor niggle is that they default to off (rather than the option of 'previous state') on boot-up.
Data point: On barakta's trike, which is a retrofitted kit, I've provided power for the existing dynamo lights (which have a sensor mode and switch on the front light in the usual manner) whenever the battery system is switched on. Normally we leave them in sensor mode. Potential gotcha is leaving the battery switched on when parking the bike (eg. at a pub), and the lights coming on at dusk.