turn on find my iphone via computer

Hi

Find My iPhone can only be turned on / set up on the iPhone itself:

Set up Find My iPhone, iPad, and Mac - Apple Support

If Find My iPhone was turned on before your iPhone was lost or stolen, then you can use a computer (or the Find My iPhone app on another iPhone, iPad or iPod touch, if you have one) to turn on Lost Mode and to attempt to locate your iPhone:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If Find My iPhone was not turned on before your iPhone was lost or stolen, then it cannot be turned on remotely afterwards, via a computer.

I believe the simplest way to do it would be going to icloud.com and signing in

you can also download the app from the Mac AppStore if you're using an apple computer