You need enable automatic authentication, see admin guide: http://onlinehelp.tableausoftware.com/current/server/en-us/config_general.htm#idd0fcdb96-5521-4a6b-8ec9-9c7912f32a61

With Active Directory user authentication, administrators can also automatically log in users based their current Windows credentials (Enable Automatic Login). This means that the user’s credentials are being passed from their local computer, not from another system or portal that they may have logged in to.

If you use Active Directory:

• You can optionally Enable Automatic Login, which uses Microsoft SSPI to automatically log in your users based on their Windows username and password. If you select this setting, you cannot also select Enable Guest later. Also, do not select Enable Automatic Login if you plan to configure Tableau Server for trusted authentication.