How to implement a Security token in a WCF soap response?

I am implementing an API from a bank and they require a security token to be provided. In the header of every soap message there is something which looks as follows:

<soapenv:Header>
   <tpw:BinarySecurityToken ValueType="MAC" Id="DesMacToken" EncodingType="Base64" Value="**xvz**"/>
 </soapenv:Header>
 

According to their documentation I need to generate an 8 byte MAC value on the body of each message. The MAC is generated by the CBC-MAC algorithm and DES as the block cipher. The contents of the soapenv:Body tag of each message is used as the data for the MAC calculation.

So my question is how do I get WCF to do this? I have put the following code together to create the MAC value, but am unsure how to get this into the header of every message.

private string GenerateMAC(string SoapXML)
         {
             ASCIIEncoding encoding = new ASCIIEncoding();
 
             //Convert from Hex to Bin
             byte[] Key = StringToByteArray(HexKey);
             //Convert String to Bytes
             byte[] XML = encoding.GetBytes(SoapXML);
 
             //Perform the Mac goodies
             MACTripleDES DesMac = new MACTripleDES(Key);
             byte[] Mac = DesMac.ComputeHash(XML);
 
             //Base64 the Mac
             string Base64Mac = Convert.ToBase64String(Mac);
 
             return Base64Mac;
         }
 
         public static byte[] StringToByteArray(string Hex)
         {
             if (Hex.Length % 2 != 0)
             {
                 throw new ArgumentException();
             }
 
             byte[] HexAsBin = new byte[Hex.Length / 2];
             for (int index = 0; index < HexAsBin.Length; index++)
             {
                 string bytevalue = Hex.Substring(index * 2, 2);
                 HexAsBin[index] = Convert.ToByte(bytevalue, 16);
             }
 
             return HexAsBin;
         }
 

Any help will be greatly appreciated.

More info:
The bank has provided a WSDL which I have used as a service reference. An example of a response that is sent:

[System.Diagnostics.DebuggerStepThroughAttribute()]
 [System.CodeDom.Compiler.GeneratedCodeAttribute("System.ServiceModel", "4.0.0.0")]
 [System.ServiceModel.MessageContractAttribute(WrapperName="LogonRequest", WrapperNamespace="http://webservice.com", IsWrapped=true)]
 public partial class LogonRequest {
 
     [System.ServiceModel.MessageHeaderAttribute(Namespace="http://webservice.com")]
     public DataAccess.BankService.BinarySecurityToken BinarySecurityToken;
 

The BinarySecurityToken (that goes in the header) looks as follows:

 [System.CodeDom.Compiler.GeneratedCodeAttribute("System.Xml", "4.0.30319.233")]
     [System.SerializableAttribute()]
     [System.Diagnostics.DebuggerStepThroughAttribute()]
     [System.ComponentModel.DesignerCategoryAttribute("code")]
     [System.Xml.Serialization.XmlTypeAttribute(AnonymousType=true, Namespace="http://webservice.com")]
     public partial class BinarySecurityToken : object, System.ComponentModel.INotifyPropertyChanged {
 
         private string valueTypeField;
 
         private string idField;
 
         private string encodingTypeField;
 
         private string valueField;
 
         public BinarySecurityToken() {
             this.valueTypeField = "MAC";
             this.idField = "DesMacToken";
             this.encodingTypeField = "Base64";
         }
 

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