'https://yourorgsite.com/api/LegacyOrder/_execute' Invoice - what to provide for legacyOrderStageInvoiceProcessingData?

Dear all,
could someone point me to the correct object definition please?
https://developer.imis.com/reference/executesmlegacyorder#invoice

{
"$type": "Asi.Soa.Core.DataContracts.GenericExecuteRequest, Asi.Contracts",
"OperationName": "Invoice",
"EntityTypeName": "LegacyOrder",
"Parameters": {
"$type": "System.Collections.ObjectModel.Collection1[[System.Object, mscorlib]], mscorlib", "$values": [ { "$type": "Asi.Soa.Commerce.DataContracts.Order.LegacyOrderStageInvoiceProcessingData", "$value": "legacyOrderStageInvoiceProcessingData Value" } ] }, "ParameterTypeName": { "$type": "System.Collections.ObjectModel.Collection1[[System.String, mscorlib]], mscorlib",
"$values": [
"Asi.Soa.Commerce.DataContracts.Order.LegacyOrderStageInvoiceProcessingData"
]
},
"UseJson": false
}

What needs to go into the legacyOrderStageInvoiceProcessingData object?
Any help is much appreciated.
Thank you!
Alexander