Matching leading digit, complementary endings

Multiply the tens digit by the next digit up, then append the product of the units digits.

1. The tens digit is $6$, so multiply $6$ by the next integer $7$ to get $42$.
2. The units digits give $4\times 6=24$, written as a block of $2$ digits.
3. Place the blocks side by side: $42$ followed by $24$ gives $4224$.
4. Check with $65^2-1=4225-1$, which is $4224$.

Answer: $4224$