To add a searx instance to your browser's search engines, visit the instance and follow the instructions on https://searx.xyz/about ("How to set as the default search engine?").

For example, to add the instance searx.xyz to Firefox, visit https://searx.xyz/ , click on the "..." icon to the right of the URL bar, and click "Add Search Engine." You can then choose searx while typing a query or can make it your default search engine (see about:preferences#search).