Select "Video > Stock" from the Library Window on the left, enter "ocean" in the search bar at the top, and search. Click on the image that you like, and it will be inserted into the Editing Window in the center at the default size.
Next, we'll insert text. Select "Text > Stock" from the Library Window on the left side, and click on the font you like. Then, in the Editing Window in the center, a "Text" element with the corresponding font style will have been inserted.
In the Editing Window, double-click on Text Element to switch to editing mode. Enter the content as "Let's protect the future of the Earth together". Once you've entered it, click outside the Editing Window to exit text editing mode.
Grab the text and position it where you want, adjusting its size. Stick it at the bottom like a subtitle, and change the size by sliding to fit the slide size.