There maybe an issue with the documentation.

From local documentation "BodyData" is there and works. The link in the previous post does say "Wolfram Language"?

The Options section of the Documentation located http://reference.wolfram.com/language/ref/URLFetch.html has "Body" as the option for the payload. I am using Mathematica version 10.0.2 and the local documentation has "BodyData" as the option for the payload. I would go with your local documentation ref/URLFetch. I did not find it per se though, the payload concept is part of most HTTP Request/Response client and server implementations.

Kay Herbert:

Try

URLFetch[
    "https://apis.tdameritrade.com/apps/300/LogIn?source=#sourceID#&version=#version number#",
    "Method" -> "Post",
    "Headers" -> {"Content-Type" -> "application/x-www-form-urlencoded"},
    "BodyData" -> "userid=#userid#&password=#password#&source=#sourceID#&version=#version number#\n\n",
    "Cookies" -> True,
    "VerifyPeer" -> True]

These variables are assigned by TD Ameritrade, hopefully you have them

• userid=#userid#
• password=#password#
• source=#sourceID#
• version=#version number#

Also assume you can fill in the proper Mathematica variables and re-work this starter code to work. So the #variable# should be replaced with the correct strings.

Hans