Click Customer > General tab > Source of Lead > 
Edit Popup Contents to display window.
1.Click 
Add to add a new row
2.Add a new label into the descriptions column, e.g. a branch name
3.Complete any extra information in the other 2 columns. These are optional so can be left blank if not needed.
Repeat by adding in new rows as needed.
Click 
Update to save and close the window