@remoteUser1 , In your case, please refer to steps 8,9,10 & 11 of this article.
1) Since, your existing virtual network is created with a single address space of 10.0.0.0/24, which is used by the default subnet, you need to add an additional IP address range to the virtual network. The size of this address range and the actual IP address range to use depends on other network resources already deployed. The IP address range shouldn't overlap with any existing address ranges in your Azure or on-premises environment. Make sure that you size the IP address range large enough for the number of VMs you expect to deploy into the subnet.
So basically, you would need to add a new address range to your Virtual Network address spaces and create a subnet out of that range.
Please refer : https://learn.microsoft.com/en-us/azure/virtual-network/manage-virtual-network#add-or-remove-an-address-range
2) How to add a VM to a host pool:
https://learn.microsoft.com/en-us/azure/virtual-desktop/expand-existing-host-pool#add-virtual-machines-with-the-azure-portal
How to add Application security group to your VM:
https://learn.microsoft.com/en-us/azure/virtual-network/tutorial-filter-network-traffic#associate-network-interfaces-to-an-asg
Kindly let us know if the above helps or you need further assistance on this issue.
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