Double click the “Start at boot”, select the check box and OK.
When there are more than one VMs need to be started at boot. I can set up the order of starting of them.
Start/Shutdown order: Defines the start order priority. E.g. set it to 1 if you want the VM to be the first to be started. (We use the reverse startup order for a shutdown, so a machine with a start order of 1 would be the last to be shut down). If multiple VMs have the same order defined on a host, they will additionally be ordered by VMID in ascending order.
Startup delay: Defines the interval between this VM start and subsequent VMs starts . E.g. set it to 240 if you want to wait 240 seconds before starting other VMs.
Shutdown timeout: Defines the duration in seconds Proxmox VE should wait for the VM to be offline after issuing a shutdown command. By default, this value is set to 180, which means that Proxmox VE will issue a shutdown request and wait 180 seconds for the machine to be offline. If the machine is still online after the timeout it will be stopped forcefully.
Regarding the LXC Container, there are the same options available.