You don't set permissions in a login script. Login scripts run in the context of the currently logged in user (there's a clue in the name) and unless your system is horribly broken you cannot grant yourself permissions to something you don't already have permission to access.
I hope that what you're actually after is only mounting a drive for a user that already has permissions to access it and this should be possible with a bit of experimentation. The exact syntax and effort need will depend on your platform, and depending on what that is, login scripts might not be the best way of doing this; in Windows I would use GPOs to share drives based on whether or not the user has permission to do so, for example.