In order to promote a Facebook video with Facebook ads you need to either be the owner of the video or have permission to crosspost it.
Here is how to establish a video crossposting relationship between two pages and get permission to post a video.
1. Setting up Crossposting
Head on over to the settings area of a Facebook page, assuming you are a page admin you will see a section called crossposting.
Enter in the name of the page you want to establish a video crossposting connection with.
At which point you should then let that the other page know that a crossposting connection has been started so an admin on the destination page can approve that connection.
Once both pages have completed this step the page that owns a video can approve that other page to crosspost the video.
2. Granting access to crosspost a specific video
The owner of the video must go to the video, click edit after hovering over the three dots to the right of the like, comment, share area.
Then go to the distribution section and turn on crossposting for the Facebook page you want to give crossposting ability to.
If you do not see the Facebook page you want to share it with in the list, it probably means the two pages are not yet finished setting up crossposting and should complete step 1 above.