Do you use a login/security to access the hub? If so, you can't do it with a simply CURL as it needs to login first. So you would need to do a GET and login, then do the POST.
Someone showed how they did this with Tasker on an Android device. While not the exact same execution in your realm, it would be the same steps - so might be worth a search/read.
I don't recall setting up any login for accessing the hub, haven't had any prompts to login from the various devices I access the hub from
Ah, then I don't know. 
Have you tried the curl command without the 8081 port? The port is only needed when sending the reboot command from the HE hub itself.
$ curl -X POST http://192.168.7.65/hub/reboot
{"message":"Hub rebooting"}
Worked for me.
Thanks, I'll give that a try when I get home. Didn't know the port was just for that... Wonder if it will work when the UI is non-responsive.
Anyone use the reboot command when the hub LED was RED?
Any thoughts of a software watchdog since the hub is running in a container? Seems like if you're changing the LED to RED you could restart the container/VM and get things running again with a crash report of some sort so mission critical things don't go down.
Hit or miss for me. It has worked a few times and not worked other times. Just be patient because it can take a while to initiate the reboot sometimes.
My hub has just died again. 
I was not doing anything at the time.
Dashboard started spinning.
Hub UI top right notification flag was red.
Clicked on it and it said zigbee was offline.
Now no response from anything. DEAD.
Time to turn on and off AGAIN!!!!!!!!!!
EDIT: Support (@bobbyD) has been super efficient and identified a possible issue with my set up.
I had disabled the Chromcast app but still had an app sending messages to the GH devices.
DOH.
Today we have been coming and going and this would generate these messages.
Disabled that app and will see what happens.
So support is telling you that the Chromecast integration they have (which is in beta) may be causing your lockups?
Yes. They identified an issue a week ago with it generating errors. I should stress this is in my setup. It MAY not be affecting everybody. As it was not used much I just disabled it.
Got my POE adapter. Just plugged it in and everything appears to be working.
This now frees me up from moving the hub much easier around the ceiling or different floors.
So you can now cycle power using a network command? How does it work?
Answer from above...(replace the IP address with your hub's, of course.)
It has to be a POST command, not a GET. Thus, you cannot do this from a simple browser window. Also, if you have enabled Hub Security, you have to log in first... This makes it a little harder.
Yeah, sure, saw that reboot command. I just wondered if there was something special about the POE beyond just powering the unit....some way to switch/activate/deactive the power via the network.
Most POE switches are manged and allow you to power cycle the port.
Nope, the Hubitat hub has no clue about POE.
I had the stink eye for the Chromecast Integration, but it's been disable for several days and today I got a "lockup" that required a reboot. I'm putting that in quotes because what many are describing as lockup, may actually be a very slowed UI or UI and hub. I was able to get to the settings page and then eventually reboot, but it took a long time to get there.
I do have errors because my rules tried to call on the Chromecast Integration and it wasn't available, but I don't have errors relating to the Chromecast Integration itself, because it wasn't enabled.
If needed. I just do a shut and a no shut on the interface if the switch
@bobbles had this and he also needed to disable them rules as well.
Download the Hubitat app
