How to get the task list item created by user name

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SharePoint2013

Hi, after creating a list item in sharepoint task list it is sending mail to creator and assigned to person, After item created through C# CSOM i am trying to update the item fields and also updating the Editor and Author fields. After successfully running program i am able to see the created by and modified by fields are updated. After task list item updated mail is sending. But in the mail content the created by field at the header section not able to update.

Please help me to solve this problem.

Please find the attached screenshots.

What I have tried:

ListItem _ToUpdateEditfld = _UpdateEditorFld[0];
_ToUpdateEditfld["Author"] = clientContext.Web.EnsureUser((usrmodifer).Title);//
_ToUpdateEditfld["Editor"] = clientContext.Web.EnsureUser((usrmodifer).Title);//
_ToUpdateEditfld["Comments"] = "";