Sharing a post can be done in two ways...
The first way is immediately after you have posted
As soon as this is done you get given the option to share automatically.
You can select from Facebook, Pinterest, Twitter or LinkedIn
Or you can copy the URL to share in other ways.
The second way is on the boost page when managing your post
Go to manage posts in the dropdown menu.
Find the post you want to boost and click the boost button.
Here you will find an option to share your post on social media or copy the URL.
How to use the Facebook sharer and post directly on your Facebook business page.
Click the dropdown menu in the top left corner and select "Share on a Page you manage"
Select the business page you wish to share your post to from the dropdown that appears
When you set up your business put in all your social media URL's