1. Go to preferences in your searx instance. In your case https://paulgo.io/preferences
2. After reviewing the settings, go to the cookies tab and copy the link under Search URL of the currently saved preferences:
3. Go to your Firefox settings in the three pointed menu
4. Go to general>Search>Manage alternative search engines>Add search engine
5. Paste the link you copied in the box called URL to use for search
6. Give it a name and Save it

Having added the search engine you can also make it the default if you want. You’ll be able to select it by taping the icon with the search engines while typing a search string.

Edit: Corrected the place to get the search string url