To get Instance name, just need to be logged to Payrexx panel. What you will see at the URL next to payrexx.com will be your instance name.
Second step is to generate API keys
Last thing to configure at the Payrexx panel is webhook.
When done, just open your GrandNode plugin configurtion panel, and paste proper values to
Do not forget to enable payment at the payments list of GrandNode.
When all done properly, test it. Payrexx is redirect type plugin, so when you click confirm button at the order, you be redirected to Payrexx payment form