If you omit the password in the command, httpie will prompt of it.
http PUT 127.0.0.1:8080/myfirstdb desc='this is my first db created with restheart' -a username
If successfully authenticated, RESTHeart returns you an auth-token that you can you use as a temporary password for further calls (it has a time to live that can be set in the configuration file).
Here an example of response headers:
Auth-Token: 6a81d622-5e24-4d9e-adc0-e3f7f2d93ac7
Auth-Token-Location: /_authtokens/user@si.com
Auth-Token-Valid-Until: 2015-04-16T13:28:10.749Z
so you can do (note the auth-token used as the basic authentication password):
http GET 127.0.0.1:8080/myfirstdb restheart' -a username:6a81d622-5e24-4d9e-adc0-e3f7f2d93ac7
Also note that you should use https in production environments.
For more information have a look at the security section of the RESTHeart documentation https://softinstigate.atlassian.net/wiki/x/W4CM