TT #107 · Parashas Shoftim · Second Aliyah · Passuk 17:18
And it shall be, when he sits on the throne of his kingdom, that he shall write for himself a copy of this Torah on a scroll, from before the kohanim, the Leviim.
Torah Temimah points to Sifrei Devarim 160.
TT lemma: וְכָתַב לוֹ.
English: ‘And he shall write for himself’ – for his own sake, that he should not write by means of an agent.
Derash on personal obligation (Sifrei) — the language ‘for himself’ is read as requiring the king’s own hand or direct responsibility.
The king bears personal responsibility for the writing of his Sefer Torah; the mitzvah is not fully discharged by mere ownership of a scroll written by others.