I believe that prayer to be a later-time addition to the Book.
Esther is (in the Hebrew version) one of only two books of the Bible that do not directly mention God (the other is Song of Songs). It is the only book of the Tanakh that is not represented among the Dead Sea scrolls. It has often been compared to the first half of the Book of Daniel and to the deuterocanonical Books of Tobit and Judith for its subject matter.
Additions to Esther
An additional six chapters appear interspersed in Esther in the Septuagint, the Greek translation, which then was noted by Jerome in compiling the Latin Vulgate; additionally, the Greek text contains many small changes in the meaning of the main text.
The extra chapters include several prayers to God, perhaps because it was felt that the above-mentioned lack of mention of God was inappropriate in a holy book. Jerome recognized them as additions not present in the Hebrew Text and placed them at the end of his Latin translation as chapters 10:4-16:24. However, some modern Roman Catholic English Bibles restore the Septuagint order, such as Esther in the NAB.
Thank you !
Thank-you for the info.