You will need to add a new series of data. This data will ultimately be used to be displayed as a target line in your chart. So the new series of data will be all the same values (the value of the target you are trying to set) see example below:

Jan Target

Week1 ____75__ 50
Week2 ____83__ 50
Week3 ____45__ 50
Week4 ____65__ 50

Add the new data to the source data for the chart. This will be displayed as an additional column/bar in your chart. Right click on one of the new columns and select chart type and then set it to a line chart type. This will give you the desired result.