To help everyone avoid confusion, I will reformulate the question in two parts.
First : "how can make an authenticated HTTP request with a browser, using BASIC auth?".
In the browser you can do a http basic auth first by waiting the prompt to come, or by editing the URL if you follow this format: http://myusername:mypassword@somesite.com
NB: the curl command mentionned in the question is perfectly fine, if you have a command-line and curl installed. ;)
References:
Also according to the CURL manual page https://curl.haxx.se/docs/manual.html
HTTP
Curl also supports user and password in HTTP URLs, thus you can pick a file
like:
curl http://name:passwd@machine.domain/full/path/to/file
or specify user and password separately like in
curl -u name:passwd http://machine.domain/full/path/to/file
HTTP offers many different methods of authentication and curl supports
several: Basic, Digest, NTLM and Negotiate (SPNEGO). Without telling which
method to use, curl defaults to Basic. You can also ask curl to pick the
most secure ones out of the ones that the server accepts for the given URL,
by using --anyauth.
NOTE! According to the URL specification, HTTP URLs can not contain a user
and password, so that style will not work when using curl via a proxy, even
though curl allows it at other times. When using a proxy, you _must_ use
the -u style for user and password.
The second and real question is "However, on somesite.com, I'm not getting an authorization prompt at all, just a page that says I'm not authorized. Did somesite not implement the Basic Auth workflow correctly, or is there something else I need to do?"
The curl documentation says the -u option supports many method of authentication, Basic being the default.
Best Answer
It seems chrome will always show you the login prompt if you include a username in the url e.g.
http://me@example.com
This is not a real full solution, see Mike's comment below.