The usual way.
At the bottom of that window there should be two buttons "OK" and "Cancel"
You did not show the whole screen
And afterwards you save the entire question with the "Save" button.
But as you use the Popup HTML editor, you are in source mode initially, and by clicking the pencil you switch to WYSIWYG mode.
Volunteers are not paid.
Not because they are worthless, but because they are priceless