The Sharing window allows you to share your posts to a linked account. If you want to share a post in a message, use the sharing feature of conversations.
To share a post:
Right-click any of your posts, then select
Share > Send to Linked Account… from the contextual menu.
When viewing the Details of your own posts, select the
Share > Send to Linked Account… item under the actions button in the title bar.
When viewing the Details of your own posts, use the
Share > Send to Linked Account… Menu Bar item.
You can provide a custom description which will accompany the shared item (if you wish to adjust the caption, see editing). Choose at least one social network to share to, and click
Send to Linked Account… item will only appear when you are sharing posts that you posted yourself.