, we have 2 different editors. Markdown editor and Standard editor.
1. When you are editing a sticky note in markdown editor, select the text and press Ctrl+B to make it bold. You can also right click inside the sticky note (in edit mode) and choose Formatting from the menu so that the bottom formatting toolbar is visible. Then you can select formatting options from the toolbar.
2. Alternatively, you can switch to the Standard editor and use classic formatting options similar to above.