This FiveM server plugin is a third party development.
This is the official documentation for it. If you are looking for a tutorial, try this FiveM tutorial here.
Put your FiveM Server IP in this field (ie: 000.000.000.000). You can use a DNS entry if you have one (ie: myserver.mydomain.com)
Put your FiveM Server Port in this field here.
Once you have filled out these settings, you need to copy the internal name of this object, then choose a button to connect with. In the actions for that button, create a new action, select FiveM and paste the name of your object into the box. FiveM tutorial here.