To find the vector element, choose "Search" or the "All" box and type in the keyword "rectangle", "box", "square", etc. or any kind of frame you have in mind. Again, drag & drop the frame you like.

Now, search for "box" in the left menu and drag & drop a solid-color box to the editing space.The box's color doesn't matter as we can change it later on.

Click on the box's corner to resize and change the color to white. Then, click "back" to move the white box backward.