How to add custom extension to existing vm using arm template - azure

Question

How to add the following custom extension to run the powershell script in already existing VM ? How to refer to existing VM ?
{
"condition":"[empty(parameters ('DR User Secret'))]",
"type":"Microsoft.Compute/virtualMachines/extensions",
"name":"[concat(parameters('vmName'),'/', 'customscript')]",
"apiVersion":"2015-06-15",
"location":"[resourceGroup().location]",
"properties":{
"publisher":"Microsoft.Compute",
"type":"CustomScriptExtension",
"typeHandlerVersion":"1.9",
"autoUpgradeMinorVersion":true,
"settings":{
"fileUris":[
]
},
"protectedSettings":{
"commandToExecute":"[concat('powershell -ExecutionPolicy Unrestricted -file ', 'C:\\test.ps1', ' -AdminPass ', parameters('Password'))]"
}
}
}

Answer 1

Here is an alternate way you may also try.
I created a VM using the ARM template.
Reference: https://learn.microsoft.com/en-us/azure/virtual-machines/windows/ps-template
And I use the below following command that uses the Custom Script extension to download a script from a GitHub repository onto the target virtual machine and then run the script.
fileUris: The locations where the script files are stored.
Set-AzVMCustomScriptExtension -ResourceGroupName "v-rash18" -VMName "SampleVM" -Name "myCustomScript"
-FileUri "https://raw.githubusercontent.com/neilpeterson/nepeters-azure-templates/master/windows-custom-script-simple/support-scripts/Create-File.ps1"
`
-Run "Create-File.ps1" -Location "Central US".
Reference: https://learn.microsoft.com/en-us/azure/virtual-machines/extensions/features-windows#run-vm-extensions
To check the extension is set on the existing VM is Get-AzVMExtension