Tortoise SVN has an option for saving authentication, but it does not work for SVN. In order to save the authentication, go to Tortoise settings, for example by right clicking in the Windows Explorer and then selecting Tortoise > Settings. In the settings window, select Network. Then add the path to Tortoise Plink (it is in the “bin” folder of the Tortoise installation) and add an option for a username and password using options -l and -pw.

For example:
“C:\Program Files\TortoiseSVN\bin\TortoisePlink.exe” -l foo -pw bar