When using either the online or app or OneNote Win10 app, adding your own text to these stickers is quite simple:
Step 1: Click Insert > Stickers
Step 2: Pick a Text Bubble Sticker
Step 3: Type Your Own
Step 4: BINGO!
Reposition if needed. The stickers will act like any other inserted image, so the method of positioning, moving, and resizing is the same.