Online TEFL courses use different platforms, so without knowing which course you're taking or having experience of that particular course, it's difficult to offer tailored advice.
Have you checked if it's a box that extends as you type? If it isn't, you need to write in a clear and concise manner, avoiding any unnecessary words, e.g., "Explain the task to the students and give them the handout" (11 words) versus "Explain task and give students handout" (6 words).
If you need to reduce the number of characters (letters/symbols), you can abbreviate some common words, e.g., students = Ss, teacher = T, whiteboard = WB, handout = HO, homework = HW, etc.
Hope that helps.