I think you are asking about your NovoRapid bolus?
The reason for pre-bolusing (taking your fast acting insulin before eating) is to attempt to match the peak of the insulin activity (Google “NovoRapid profile” to see the graph of the insulin activity over time) with the peak of the digestion of the carbs in your food.
It is common to be advised to bolus 10 to 20 minutes before eating but in reality, different food is digested at different rates (we are told about fast acting sugars which we use to treat hypos and, at the other end of the scales are carbs eaten with high fat meals where the fat slows digestion).
If we bolus too late, we see a higher spike on our Libre and, if we bolus too early, we see a dip which may become a hypo.
Over time, we could work out different pre-bolus times for different meals. But 15 minutes is a good “all rounder”.
However, when eating out, I will bolus as the food arrives. I can not sit there watching my food go cold for 10 minutes and it is too risky to guess exactly what will be on the plate and when it will arrive.
So, to answer your question, you can inject as the food is delivered but expect a BG spike if you do so.