Hmmm, a little more tricky if you go through a signon screen...but it can be done 
.
If you have multiple password adapters configured, you must include the password adapter as a parameter to the login URL.
Try this, It will prompts for UserName and Password, then goes to the project or document.
The format of this non single signon (SSO) logon webjumper link is:
Where
mycompany.ariba.url.com is your specific system url
PasswordAdapter2 is the specific password adapter used for this user
8lofgo.4 is the objectID of the project or document (much tricker to get this but can be gotten through Inspector)
There are some Parameters.table settings involved too - search for Paramaters/values = 'webjumper'
There is not much details in the DOcumentation...More info can be found in
9R1 Ariba Upstream Platform Configuration Guide (Most info is here)
9R1 Ariba Upstream Platform Release Guide
9R1 Upstream Platform Configuration Guide