To schedule the first comment on an Instagram post:
Select an Instagram account and add an image to your post
Enable the First comment option
Type your first comment (2,200 characters and 30 hashtags max - note that the hashtag count includes both hashtags in the post AND in the first comment)
The first comment appears in the post preview (and in the shared calendar too, if you use this feature)
The first comment will be published at the same time as your post