Add A Signature Line - Microsoft Support

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You can underline blank spaces for a printed document by pressing the TAB key and applying the underline formatting to the tab characters.

Apply underline formatting to tab characters

On the Home tab, in the Paragraph group, click Show/Hide.

The marks that indicate spaces and tab stops are displayed.
Press the TAB key.
Select the tab characters that you want to underline. A tab character looks like a small arrow.
Do one of the following:
- To apply a simple underline format, press CTRL+U.
- To apply a different style of underline, on the Home tab, in the Font group, click the Font Dialog Box Launcher, click the Font tab, and then select a style from the Underline style list.