In the new window click “Import XML" button and select the XML file from your computer.
After you import XML file all your WordPress post or Pages will appear with their respective link preview on the left side. On the right you will be able to delete messages you don't want to add to the group by clicking trash icon. After deciding which messages you want to add click ''Save" button. The messages will be imported to the snippet group so you can use them in the future.