XenServer 7.3 changed the way auto startups worked.Â
You first need to get the POOL id:
xe pool-list | grep uuid
uuid ( RO) : 5e9cda02-a5e0-ee75-26fe-caba7b4dfe04
Next you will need to enable auto startup on the pool master.
xe pool-param-set uuid=5e9cda02-a5e0-ee75-26fe-caba7b4dfe04 other-config:auto_poweron=true
Now that you have turned on the auto-start feature, you can enable individual machines to auto start.
xe vm-list
uuid ( RO) : 017126c8-dc39-7c10-f0f2-886a8267458c name-label ( RW): win10 power-state ( RO): running
enable the uuid of the individual VM you want to startup on boot.
xe vm-param-set uuid=017126c8-dc39-7c10-f0f2-886a8267458c other-config:auto_poweron=true
Start Order and Start delay
Now you can set the start order and delay in XenCenter.

Conclusion
The auto start feature is disabled because of a HA environments. If your using a standalone XenServer this allows you to configure settings in Xen for auto start up. Don’t forget to backup of your VM’s.
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