Invalid login on web, but works on API Reference "Try it"

[TimSHC]

I am trying to authenticate to the Authorize.Net sandbox.  When I go to the API Reference on developer.authorize.net and use my API login and Transaction Key credentials, it works fine.  When I try to post it via the web it gives me an error of  "The merchant login ID or password is invalid or the account is inactive".  The command I am entering is: 

https://test.authorize.net/gateway/transact.dll?<authenticateTestRequest xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
<merchantAuthentication>
<name>xxxxxxxx</name>
<transactionKey>xxxxxxxxxx</transactionKey>
</merchantAuthentication>
</authenticateTestRequest>

What am I doing wrong?

[RaynorC1emen7]

you can't pass the xml as a param.

and that the wrong url for using xml

from api endpoint tab

http://developer.authorize.net/api/reference/index.html#authentication

https://apitest.authorize.net/xml/v1/request.api