Log onto your Plesk Account.
On your Dashboard, click on 'Website & Domains' on the left-hand side of the page, unless you are already on it.
On the right-hand side of the webpage, click on 'ID Mail'. This will take you to the webmail site.
You can also click on 'Open in Plesk' or 'Open in New Window' beside ID Mail in the info box of a specific site.