how to create a Azure Spot VM using Azure SDK

You need to create them as you normally would, but specify the eviction policy for the vm. That would be the way to create spot instance

Here is a sample code to create a Azure Spot VM

/// <summary>
/// Create a Spot VM
/// </summary>
/// <param name="vmSizeName">Name of the size of the VM</param>
/// <param name="spotBiddingMaximumPrice">Specifies the maximum price you are willing to pay for a Azure Spot VM/VMSS (use -1 to match on-demand pricing)</param>
/// <param name="deallocateOnEviction">If set, when the Spot VM is evicted, it will be automatically deallocated (recommended!).</param>
/// <returns>VMProperties instance</returns>
public static VMProperties CreateSpotVM(string vmSizeName, double spotBiddingMaximumPrice = -1, bool deallocateOnEviction = true)
    => new VMProperties()
    {
        Hardware = new VMHardwareProfile(vmSizeName),
        SpotVMBillingProfile = new VMBillingProfile(spotBiddingMaximumPrice),
        SpotVMEvictionPolicy = (deallocateOnEviction ? "Deallocate" : null)
    };