Using New Outlook on Windows for personal email, calendar, and contact management
I figured out how to do it.
Go to Microsoft Word and create the horizontal line there, then copy and paste that line into your outlook signature.
STEP BY STEP: In Word, on the Home tab, select the down arrow next to Borders. Select Borders and Shading. In the dialogue box that pops up onscreen, you can then manipulate the color and thickness of your line too. On the Borders tab of that dialogue box, I clicked on Custom so that my line does not become a full border that wraps around the full document. Also, the width I picked was 4 1/2 pt.
*If you just need a basic black line, you can type out three lower dash lines (this thing _ on your keyboard). You have to use the Shift key to access the bottom dash (this is not it's official name sorry). Click on it 3 times ( ___ ) and hit Enter. That creates an underline in both Word and Outlook.
Hope this helps you. It shouldn't be this hard!