The original manuscripts wouldn’t have it, if we had them to see.
No scholar seriously suggests that the phrase “for Thine is the kingdom …” was in the original prayer of Jesus. Could it have been said by Jesus? Yes, it could have, then we would have to assume the Apostles used it in order for us to have it, but the Gospel accounts do not include it so no one makes that claim.
The phrase is strictly used in eastern liturgies, by the priest, which is why there is evidence for it in the Didache (a very early catechetical tool). It is clearly very old and has been used continuously since before the Didache was written, but is part of tradition, not recorded in the Gospel accounts.