1. Go to your settings
2. Scroll down or search for 'On Startup'
3. Select 'Open a specific page or set of pages'
4. Then 'Add a new page'
5. Then add: https://randomneocities.neocities.org/
On Firefox:
1. In the top right corner of the window, click the hamburger icon. 
2. Click on 'Options' and go to 'Home'
3. In the bar next "Homepage and new windows", input: https://randomneocities.neocities.org/