Searching about trying to find if there was an equivilant mailto: for creating new emails in Outlook Web Apps – could we find anything! Nope! After some investigating we found out that is you add the following AFTER you OWA url it will open a new window with a new email message much like mailto: works with Outlook.
<insert OWA URL here>/?ae=item&a=new&t=IPM.firstname.lastname@example.org
Not very complicated but solved our problem, so thought we would share it.