1. Open the word document and click the Start button on the top toolbar:

2. Find the underline function button, whose icon is "U" with a horizontal line below it, click the icon, and then place the cursor at the position where you need to add a horizontal line:

3, hit the space bar on the keyboard, then the horizontal line will come out, you can control the length of the horizontal line according to the number of times you hit the space bar: