CommentsController.cs

[HttpPost]
public async Task Post([FromBody] Comments comment)
{
    Guid uuid = Guid.NewGuid();
    var item = new Dictionary<string, AttributeValue>()
    {
        {"id", new AttributeValue{S = uuid.ToString()}},
        {"username", new AttributeValue{S = comment.Username}},
        {"comment", new AttributeValue{S = comment.Comment}},
        {"postId", new AttributeValue{S = comment.PostId}},
        {"date", new AttributeValue{S = DateTime.Now.ToString()}}
    };

    PutItemRequest request = new PutItemRequest
    {
        TableName = TableName,
        Item = item
    };
    await _amazonDynamoDb.PutItemAsync(request);
}