The horizontal line is one of the line tools on MT5, if you want to add a horizontal line on the MT5 chart, you can follow the steps below.

Step 1: Click "Insert" in the MT5 menu.

Step 2: Click "Objects" - "Lines" - "Horizontal Line" in the drop-down menu, and then click in the chart.

Or click on the horizontal line tool in the MT5 toolbar and click on the chart.