YOURLS will need the URL where you send queries for shortening URLS and your YOURLS signature token. The YOURLS signature token can be retrieved from your YOURLS administration pages. Additionally, YOURLS supports a few different options within XPoster for how short URLS are created.
They can use the post ID (which may look like https://your.ls/7328), they can use a custom keyword (that you create in the XPoster metabox when you create the post), or they can use the YOURLS default URL generation system, which uses sequential numbers.