You can't write it directly in the form. You can hand-write your signature on paper, take photos and cut them into pictures, and insert them into forms. If you really want to sign the form directly, you can press CTRL+ALT+A to take a screenshot in the blank space of the form (to log in to QQ or WeChat), and then select a brush in the screenshot to write. Then click Finish, press Ctrl+V to paste into the form, and then adjust the position, as shown in the figure.