ServiceDesk Plus Cloud Add-On is available for Outlook (Cloud version) and Exchange 2016 and 2019 (On-premises version). It is a free add-in, supported in Office 365 web-access and in the installed client as well. You can get the add-in from the Microsoft store.
For users operating with Exchange 2013, download manifest_sdp.xml file from the attachment section and upload the file to your Exchange Admin Centre for the add-in to be installed.
In Exchange Mailbox, select the icon and select Manage add-ins as shown in the screenshot below.
In the Manage add-ins page, click the + icon and select Add from a file. Upload the manifest_sdp.xml file you downloaded earlier. Click Save.