var request = new CreateBuildWithCustomContainerRequest()
{ 
    ContainerImageReference = new ContainerImageReference()
    {
         ImageName= "testimagename",
         Tag= "0.1"
    },
    ContainerFlavor = ContainerFlavor.CustomLinux,
    BuildName = "testbuildwithvmstartupscript",
    VmSize = AzureVmSize.Standard_D2as_v4,
    MultiplayerServerCountPerVm = 3,
    Ports= new List<Port>
    {
        new Port()
        {
            Name= "port",
            Num= 123,
            Protocol = ProtocolType.TCP   
        }
    },
    RegionConfigurations = new List<BuildRegionParams> { new BuildRegionParams()
    {
        Region = "EastUs",
        StandbyServers = 3,
        MaxServers = 6

    } 
    },
    VmStartupScriptConfiguration = new VmStartupScriptParams()
    {
        VmStartupScriptAssetReference = new AssetReferenceParams()
        {
            FileName = "vmstartupscriptassets.zip"
        }
    }
};
var result = await PlayFabMultiplayerAPI.CreateBuildWithCustomContainerAsync(request);

VmStartupScriptConfiguration = new VmStartupScriptParams()
{
    VmStartupScriptAssetReference = new AssetReferenceParams()
    {
        FileName = "vmstartupscriptassets.zip"
    },
    PortRequests = new List<VmStartupScriptPortRequest>()
        {
            new VmStartupScriptPortRequest()
            {
                Name = "port0",
                Protocol = ProtocolType.TCP
            },
            new VmStartupScriptPortRequest()
            {
                Name = "port1",
                Protocol = ProtocolType.UDP
            }
        }
}

PF_STARTUP_SCRIPT_PORT_COUNT

PF_STARTUP_SCRIPT_PORT_INTERNAL_0
PF_STARTUP_SCRIPT_PORT_EXTERNAL_0
PF_STARTUP_SCRIPT_PORT_NAME_0
PF_STARTUP_SCRIPT_PORT_PROTOCOL_0

PF_STARTUP_SCRIPT_PORT_INTERNAL_1
PF_STARTUP_SCRIPT_PORT_EXTERNAL_1
PF_STARTUP_SCRIPT_PORT_NAME_1
PF_STARTUP_SCRIPT_PORT_PROTOCOL_1