I need to set the Notes field to the Notes field value in other entity in CRM 2011 form. So, I need to know how get and set the Notes field using Javascript. And I'm not able to get the name of Notes field inside the section as you can seen in the below image.
OK there are two approaches required here.
@Philip_Rich pointed out that annotations for existing records are created as soon as they lose focus and can therefore be queried. You should be able to find existing code for this quite easily (if not, ask here).
You yourself acknowledged that for new (unsaved) records, annotations are not saved until after the parent record is saved. There is no supported way to access the value in the notes field at this point, however this nasty bit of code should get the value you seek. Beware that since it is unsupported, it is vulnerable to DOM changes in the forms:
var myNotesText = document.getElementById("notescontrol").contentWindow.document.getElementById("NotesTable").children[1].children[4].children[0].innerText;