SOAP API v5 Requirement Custom Properties that are Lists

Hello,

By trial and error, I figured out that RemoteArtifactCustomProperty.PropertyNumber refers to the project-property specific ordinal value.

And I have successfully insert string values to custom fields like this:

RemoteArtifactCustomProperty lob = new RemoteArtifactCustomProperty();
lob.PropertyNumber = 1;
lob.StringValue = "Liability";
req.CustomProperties = new RemoteArtifactCustomProperty[] { lob };

But I can't figure out how to insert a value when the property is bound to a List.
Using the above example, assuming that the value listed is a valid value on the List that the CustomProperty uses: How do I insert a value?

Thanks in advance,
Will Menton