Maybe you buy school lunches, or your job serves you something every day and you're not sure what it's going to be ahead of time.
Depending on how consistent those meals are, the easiest thing to do is just estimate the nutrition you'll receive during that meal. For example, school lunches are typically required to be between 750 and 850 calories, probably with a lot of variation in the macronutrients.
With this in mind, create a new "custom food" with your estimated values (do your best with the macronutrients - don't worry if they're a little off), then set it to recur every day during that meal so the generator has something to plan around. If you later find out the exact nutrition of the meal, try to account for it in your dinner/snacks later in the day (either mentally or logging stuff directly on the site). You might not hit your macros 100% accurately, but being off by 100 calories or 10 grams of something isn't going to set you back terribly, and you'll be much better off than not tracking at all.