You will pay USD to purchase the JPY amount of money you plan to purchase whatever you are buying in JPY. You would only need a FORWARD FX contract if you are planning the purchase out for a certain time period. If you are buying today, you would just enter a spot contract to purchase JPY with USD and then make your purchase. I would call whatever broker you are using to discuss your options with them.

I have a fund with a base currency of usd, but am making a purchase in jpy. I need to convert some usd to jpy to make the purchase but want to hedge against currency risk by buying an fx forward contract. In which currency should I buy the forward?