It is not possible to set a default repository on the "Save to Laserfiche" button in the Outlook plugin. You can have the plugin remember login credentials to speed up the process, but a repository must be selected. 

Note that if Laserfiche is open, the "Save to Open Folder" in Outlook will save the selected email directly to the repository, using the location and login credentials used by the client. This bypasses the login step completely from the "Save to Laserfiche" button.