There is no such option though it should already be automatically joining known networks (it has no problem doing so for my broadband/school wifi).
The hotspot on iPhone is always on so there's no need to activate it again.
The problem is the MacOS cannot auto-reconnect to the iPhone hotspot after MacOS wake up from sleep.
This is what mine looks like using High Sierra 10.13.4.
If I check that. Unplug my home router, put my iMac to sleep, and then wake it up it connects to my iPhone. Can't attest to its reliability.
With my Hotspots OFF on my iOS devices if I view Wifi network drop down I see this.
Other wifi networks are listed below those including my home network. Those are my devices signed into my iCloud that are within range and have hotspot functionality.
I don't know why you wouldn't get the ability to automatically join. Do you have the automatically join feature with actual networks?
Could be something to do with the "Instant Hotspot" feature, or lack thereof. I don't have a way of testing this since I would need to sign out of iCloud to do it. Instant Hotspot is one of Apples Continuity features. It requires the devices to be signed in with the same AppleID, Bluetooth to be on (this is how it can show battery life, LTE reception, and activate the hotspot remotely), Wifi to be on, and probably some other stuff.
Doubt any of that helped you but good luck!
