Hi Julia,
As per checking, when using Microsoft Family Safety, there isn't currently a way for a child account to proactively request additional screen time before the limit is reached. The Ask for more time option normally appears when the screen time limit has been reached or when the device becomes locked due to the limit.
As an alternative, you can contact the family organizer (parent/guardian) before your time runs out and ask them to adjust your screen time schedule or grant additional time through the Family Safety app or website.