C# XML Element Prefix SOAP

I have seen many SO answers but non seem to help with the issue I have. What I am doing is calling a SOAP service from third party and it actually requires a prefix on element names. Example:

<pref:test>test</pref:test>

I have created a client using Add service reference in VS. I have a class which is where i want the prefix on each property.

public class WebsiteSoapHeader : System.Web.Services.Protocols.SoapHeader
	{
		public WebsiteSoapHeader()
		{
			xmlns.Add("test", "https://xxx/service");
		}

		[XmlNamespaceDeclarations]
		public XmlSerializerNamespaces xmlns = new XmlSerializerNamespaces();
		[XmlElement(ElementName ="test",Namespace = "https://xxx/service")]
		public string testprop { get; set; }
	}

As this does not get serialized manually it only produces

<test xmlns="">test</test>