When commenting on your own Facebook page, you will default to commenting as the page.
To comment on other Facebook pages post as your Facebook page, follow these steps:
Note: The steps below are applicable for Android, PC, and iPhone.
Go to the page and find the post you want to comment on.
Now, hit your profile picture beside the Share (if you’re on a PC) or above the Share (if you’re on an Android or iPhone).
Select the Facebook page you prefer.
That’s all there is to it. You can now comment on the post as the Facebook page you’ve selected.
Note: If you cannot find the profile picture mentioned above, then you need to find that post of the page in the Home tab or use the search to find it. There you can find the profile picture mentioned above.