How to activate find my in iPhone using mac

It’s not clear what you’re asking.

Are you trying to add a device to Find My? The article linked above explains how to do that.

Are you somehow trying to add a device to Find My that is not in your possession? Why would you be doing that?

Are you trying to use Find My to locate someone else’s device?

The device must be physically in your possession in order to activate Find My iPhone. It cannot be turned on remotely.

Not sure what you are asking here, but the Find My option must be enabled manually on the phone itself. The Find My option cannot be enabled from another Mac, iPhone or iPad.