What was Israel's king to write for himself when he took the throne?

Answer: A copy of the law

Seated on his throne, the king was to write a copy of this instruction on a scroll in the presence of the Levitical priests.

“When he is seated on his royal throne, he must write for himself a copy of this instruction on a scroll in the presence of the Levitical priests.”— Deuteronomy 17:18 (BSB)