Open the sheet you want to add text to.

Select the "Add Line" tool in your toolbar and click on the sheet to indicate the start point of the line.

Specify the length required by inputting the value in mm via your keyboard. And the click on the sheet again to complete the line. Press esc to deactivate the tool, or continue to draw another line.

Select the line and the properties like colour, style, arrow head will appear in your design panel on the left side. You can make any changes you want there.