[SalesForce] Create chatter post with mention for API version 37.0

It's always helpful if you post images of your flow for this kind of troubleshooting. I'm assuming that you're using a record create on the feed item. Rather than this, please use the standard "Post To Chatter Action."

In the following example, I lookup my user record and then use the post to chatter action to post dynamically to my user record and @mention.

However, the issue that you're having here is that you're creating a record and then trying create a feed item. When you create a feed item, it will create it as plain text. Chatter posts that need at mentions must go through the streaming API, which the standard post to chatter action does. For this specific use case, you will need to create an apex plugin that does this work for you.

Here's some sample code, which I adapted from an example for posting files:

// Define the FeedItemInput object to pass to postFeedElement
ConnectApi.FeedItemInput feedItemInput = new ConnectApi.FeedItemInput();
feedItemInput.subjectId = 'me';

ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();
textSegmentInput.text = 'Check out this link.';

// The MessageBodyInput object holds the text in the post
ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
messageBodyInput.messageSegments.add(textSegmentInput);
feedItemInput.body = messageBodyInput;

// The FeedElementCapabilitiesInput object holds the capabilities of the feed item.
// For this feed item, we define a link capability.

ConnectApi.LinkCapabilityInput linkInput = new ConnectApi.LinkCapabilityInput();
linkInput.url = 'https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_connectapi_input_link_capability.htm';
linkInput.urlName = 'Doc link';

ConnectApi.FeedElementCapabilitiesInput feedElementCapabilitiesInput = new ConnectApi.FeedElementCapabilitiesInput();
feedElementCapabilitiesInput.link = linkInput;

feedItemInput.capabilities = feedElementCapabilitiesInput;

// Post the feed item. 
ConnectApi.FeedElement feedElement = ConnectApi.ChatterFeeds.postFeedElement(Network.getNetworkId(), feedItemInput);