cURL request - 400 Request couldn't be deserialized

I'm trying to use a cURL command to post to a form endpoint, but keep getting a "400 Request couldn't be deserialized" error. Please let me know if you see anything that's amiss with my test script below:

curl --user "APITest\API.User" --header "Content-Type: application/json" --request POST --data '{"emailAddress":"george.washington@america.com"}' https://secure.p04.eloqua.com/API/REST/2.0/data/form/18

== Info: upload completely sent off: 856 out of 856 bytes

<= Recv SSL data, 5 bytes (0x5)

0000: .....

<= Recv header, 31 bytes (0x1f)

0000: HTTP/1.1 403 Permission Error