How to turn on find iphone on another iPhone

Find My has to be activated on the device itself.

Set up Find My on your iPhone, iPad, iPod touch, or Mac - Apple Support

You cannot turn on the Find My option remotely.

Find My is what allows you to access the iPhone and track it. Having be able to access it without it being on, kind of de3feats the purpose of the option existing in the first place or being turned off.