"Subscript"

"Subscript" is a front end token that creates a SubscriptBox and fills the base with the current selection.

This token is equivalent to the menu command Edit Expression Input Subscript.

"Subscript" will fill the base with the sub-expression to the left of the selection if it is an insertion point.

To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].