Click anywhere on the white area around the chart and open the presentation properties tab. In the chart elements group, you can control the source description with the option called "Copyright." Click on the "Edit" button.
Here, you can type your preferred text. You may also want to use the dynamic text option to automatically add the source of the series.
If you don't see your new source description at the bottom right-hand corner of the screen, you may need to change the text color to a darker color. Click around the area where the Macrobond logo was before until you select the box where the source description should go. Once selected, you can change the text color in the presentation properties tab.