In this example, a numeric slider will be used to create a horizontal line on a visualization that can be moved with the slid 

1. Add a text area

2. Right click on the text area and select 'Edit text area' to toggle into Edit mode

3. Click the Insert Property control button in the toolbar (second from the right)

4. Select 'Slider' from the drop down list

5. Click the New button

6. Name the property control (don't use spaces)

7. In the 'Set property through' drop down, select 'Numerical range'

8. Set the min and max values, as well as the value interval

9. Click OK

10. Go to the Properties dialog of the visualization and go to the colors menu

11. Click the Add button and select 'Horizontal line' or 'Vertical line' and 'Straight line'

12. Choose the custom expression radio button and click the Edit button

13. Find the property in the middle list of properties and click Insert property
14. Click OK and click OK

The slider property control is now connected to the visualization