You have to use the MentionSegment in order for the @Mention to work. See this page for an example: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/connectapi_examples_post_feed_element_mention.htm

You have to use the MentionSegment in order for the @Mention to work. See this page for an example: [https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/connectapi_examples_post_feed_element_mention.htm][1]

ConnectApi.FeedItemInput feedItemInput = new ConnectApi.FeedItemInput();
ConnectApi.MentionSegmentInput mentionSegmentInput = new ConnectApi.MentionSegmentInput();
ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();

messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();

mentionSegmentInput.id = '005RR000000Dme9';
messageBodyInput.messageSegments.add(mentionSegmentInput);

textSegmentInput.text = 'Could you take a look?';
messageBodyInput.messageSegments.add(textSegmentInput);

feedItemInput.body = messageBodyInput;
feedItemInput.feedElementType = ConnectApi.FeedElementType.FeedItem;
feedItemInput.subjectId = '0F9RR0000004CPw';

ConnectApi.FeedElement feedElement = ConnectApi.ChatterFeeds.postFeedElement(Network.getNetworkId(), feedItemInput, null);

The feedItemInput.subjectId is where you set the parent ID. [1]: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/connectapi_examples_post_feed_element_mention.htm