PowerPoint doesn't have an automatic footnote creation tool, but you can add these references to slides manually. Create the footnote number in a slide and then add its text in the footer area.
Insert the Footnote Number
Go to the place in the slide where you want to add the footnote number. Type the number and select it.
Select the arrow in the corner of the Font area on the Home tab to open the Font menu. Check the Superscript box and select OK.
Add a Footer and Footnote Text
Open the Insert tab and select Header and Footer. Check the Footer box. Type the same number you added to the slide, followed by the footnote text.
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Select Apply to create the footer in the current slide. If you don't want the footnote to appear on the Title slide, check the Don't Show on Title Slide box. Don't select the Apply to All option unless you want the footnote to show on every slide in the presentation.
Format the Footnote
The footer box doesn't resize automatically to fit text. If you've inserted a long footnote, it may not fit on one line unless you make the box bigger. Select the footer box and use the points on the sides to drag it to size.
PowerPoint centers text in a footer by default. To change the position of the footnote text in the box, select it. Open the Home tab and select one of the Align Text Right, Align Text Left or Justify buttons in the Paragraph area.
When you're happy with the footer's format, change its number to superscript. Select the footer and highlight the number. Open the Home tab and select the arrow in the Font area. Select Superscript and then OK to apply the change.