When does the mounted combatant choose whether the attack hits him or his mount?

Question

For the Mounted Combatant the PHB 168 says that the mounted character can redirect the attack to himself rather than the mount. Does this need to be declared before or after the attack roll/dmg roll?

Answer 1

You can force an attack targeted at your mount to target you instead.

Choosing a target is the first step of the Attack action and it happens before any roll (see PHB p. 194). Therefore, this needs to be declared before the attack roll.