API issue of core_v2_conversation not getting messages in USer profile message box

Those two calls follow the same code path (the subject is just ignored when the older version of the API is called).  Are there any exceptions logged when its not working for you?  What are you getting back in the conversationResponse?

Update:  Tried again Widget API is working only when I tried to send message to another user except for Users are sending message to themselves but in my scenario, When Users want to update their accounts from MSD to AAD or vice versa so they will get a user token to Map their account as a message in the mailbox. But with this API, not able to get messages.
Code is below:-

#set($accessingUser = $core_v2_user.Accessing.Username)

#set($conversationResponse = $core_v2_conversation.Create( $body, $accessingUser))

Update:  Tried again Widget API is working only when I tried to send message to another user except for Users are sending message to themselves but in my scenario, When Users want to update their accounts from MSD to AAD or vice versa so they will get a user token to Map their account as a message in the mailbox. But with this API, not able to get messages.
Code is below:-

#set($accessingUser = $core_v2_user.Accessing.Username)

#set($conversationResponse = $core_v2_conversation.Create( $body, $accessingUser))

I don't think we ever intended a user to create a conversation with themself, so I am not suprised that does not work. You might consider creating your on velocity method to send the user token to the user.  In that method you could use the RunAsUser api to send the conversation from an administrator account to the user.