Just google the company name + logo, so e.g. "Nike Logo", and then go to Images. Alternatively, you can also look for the company on LinkedIn, or other social media channels.
If you search on Google, you will probably find a lot of different images. Optimally, choose one that has square dimensions (On most social media pages they are). If it has a transparent background (You can see that when it has white and grey tiles in the background), choose one that fits best on a white background.
Now, right-click on the photo, and then select 'Open image'. It now shows up on a blank background. The url in the top bar is the url we need! Ideally, it has something like ".jpg", ".png" or ".svg" at the end.
Just copy this address and paste it into the form! If you want to check again if you have the right url, just open it in a new browser window. If the correct image comes up, you did everything right!