Request body to post a new comment using REST API
I've solved it by these below

{
    "entity_id": [{
        "target_id": 612
    }],
    "entity_type": [{
        "value": "node"
    }],
    "comment_type": [{
        "target_id": "discussion"
    }],
    "field_name": [{
        "value": "discussion_comments"
    }],
    "subject": [{
        "value": "Goodbye World"
    }],
    "comment_body": [{
        "value": "comment which you want write"
    }]
}