Go to https://www.youtube.com in your computer's web browser. This is where you'll find the background music or sound for your presentation. If the computer on which you're presenting doesn't allow you to access YouTube videos, you'll need to download your preferred video and upload the video to Google Drive. You can then skip to the next part. It's at the top of the page. Type in the name of the video you want to use, then press ↵ Enter. For example, you might search for "free music for one hour" for a copyright-free music loop. Scroll through the video results, then click the video you want to use. This will open the video and prompt it to begin playing. It's best to pick music that has a simple lo-fi loop rather than a traditional song or orchestral piece. This will ensure that your music is easier to synchronize across your presentation. Once the video opens, highlight the address at the top of the window, then press Ctrl+C (Windows) or ⌘ Command+C (Mac) to copy the address.

Summary:
Open YouTube. Click the search bar. Search for a video. Select a video. Copy the video's URL.