I have the same question. Is there a way to add a custom field to a contact through the XML API?
I found the answer to my own question. The format for the custom fields is <CustomFieldX> where X is the custom field number desired.
Hey Tanya,
Glad you were able to find what you were looking for. For anyone else who stumbles upon this post you can find the information on how to achieve this by following the directions on this post.