I am creating a windows node and registering it to an already created ELB . What is the PowerShell equivalent of this command :
aws elb register-instances-with-load-balancer --load-balancer-name <load-balancer-name> --instances <instance-id>
It's Register-ELBInstanceWithLoadBalancer.
FYI, you can use Get-AWSCmdletName to do a lookup of the PowerShell cmdlet corresponding to an AWS CLI command (amongst other things):
Get-AWSCmdletName -AwsCliCommand "aws elb register-instances-with-load-balancer"
(the 'aws' part of the cli command can be omitted for typing convenience).