Add Text to connector
Select and right click on connector -> click Edit Style
Click on tab "Text"
If text is required to be on TOP of connector line change Spacing to borders -> Bottom (increase the number) 0.30 works for me. You can play with these numbers and Text anchor for required result
Result should look good :)
As an alternative to this procedure: You can as well use generic Text (F2 key) block and put text anywhere in drawing.