Hello,
I see... this feature is not available out of the box, you will need to implement it.
You can find inspiration in the Event booking system "add to outlook" functionality in
\CMSModules\EventManager\CMSPages\AddToOutlook.aspx.cs
and modify the default UI control at
\CMSModules\ProjectManagement\Controls\UI\ProjectTask\Edit.ascx
Hope this will help.
Regards,
Zdenek