Here's how to find the post ID on Facebook and Instagram:
Facebook: Post ID

1. go to business.facebook.com
2. under Business Settings, go to Page Posts
3. select the correct Facebook page
4. Under post details you will find the post ID
Instagram: Post ID
1. Go to Instagram profile
2. Click on the corresponding post

- Click on the three small dots in the upper right corner
- Click on the "Link" button in the middle
Copy the link of your post e.g.
https://www.instagram.com/p/CK_NCZPJiOxMGSO7Rv3SVMYXBcrixuPUlZrebc0/?igshid=YmMyMTA2M2Y=
The text in bold is your post ID.