LnkShortener is working out of the box. Simply share a link via the share function of your browser or any other app with LnkShortener and it will instantaneously display a shortened URL. If you have a workin polr installation enable the API and obtain an API-Key. Open the Settings in LnkShortener and enter the address of your Polr server (e.g. https://1n.pm) including the protocol (e.g. https://). Afterwards copy and paste the API-Key to the next field and you're ready to go with your own domain.