If you click on options menu (right top corner) you can select from multiple post settings.
Save as Ad, this will save the post to Facebook as unpublished (also known as a dark post), it can be used for ads and it will be visible in Facebook Ads Manager. After performing this you have the following options to find it in Ads Managers.
Option number 1:
Go to Ads Manager, click on the right top corner, hover over the "All Tools" at the bottom and click on Page Posts:
You will see there your post. Select it, click on Actions at the top of the page and hit create ad:
Option number 2:
Go to Ads Manager, create an ad, choose "Use Existing Post" but instead of selecting a post, click on "Enter post ID"
You will find the post id if in Planable you click on the activated "saved as ad" button:
and copy paste the post id from the URL:
Copy paste the id in Ads Manager and the post will appear: