For populate destinations, you need to book and ticket the outbound reward in order to hold/get it. Then later book the return when it opens.

This is easiest if your a Premier 1K or Plat as you can change or redeposit the rewards for no fee. But with low or no status - you really don't have an option.

Book something and I believe you can change the routing if origin and destination remain the same.

Too hard to get reward travel - book and ticket when you see it.