I am guessing this is for the AI scripts?
When Japan goes to war with the USA, it does not necessarily need to attack Manila on the first turn. It is fairly unlikely that the USA would ever be able to deliver supply there. An aggressive American player might reinforce the Philippines via a risky Return to Base move with an Elite/White-Print INF perhaps, but that is rare.
What Japan can do is focus on seizing it's outer defense perimeter first. The landings on the surprise impulse are precious; I can't recall ever committing 2 MAR and 2 divisions to the initial attack on the Philippines. I would commit that type of force to subduing the NEI as quickly as possible.
Japan can't ignore the Philippines for long however, or the risk of a sneaky American reinforcement convoy starts to grow. As long as they get ashore the easy way, via the surprise impulse, they can work on taking Manila on the 2nd or even 3rd turn of war with the Americans. Japan should look at the conquest of Manila holistically - planning for a land, sea (solid blockade to keep the USA away) and air campaign - perhaps a bit late in each turn, a couple CVs can go for the low odds chance of flipping the MacArthur HQ, and just wait for success on that, while they slowly reinforce the initial landing with their transport assets used farther out in the perimeter on the surprise impulse. Some of the low factor IJA bombers should also regularly be making runs over Manila as well. (I can't imagine playing the game with unlimited Re-Org; in my experience once MacArthur is flipped, he stays flipped.)
So I usually just land on the resource hex and maybe also north of Manila if I am really well supplied with divisions. [On the regular Asian scale paper maps, Japan has the option to walk Marine units across all-sea hexsides from Formosa, a good way to appear in northern Luzon while saving a transport move.]
The American defense can exploit that rather obvious Japanese strategic approach via a low risk option of putting the Philippines TERR in the hex south-east of Manila. This is a Jungle hex, good for defense; the TERR will always be in supply in that hex and probably would never be attacked directly by Japanese MARines landing on it, though they easily could (the Dutch would be grateful). This would also put a 1 strength notional defense on the resource hex, doubled to 2 in the mountains, which would require more Japanese force than I would want to use, as noted. The TERR could be withdrawn to Manila if/when the Japanese appear in force in southern Luzon though not of course if the USA took a naval impulse...