Is it possible to call a config node function from inside of my custom node?
Thanks in advance
If the config node is connected to that custom node, yes,
node.property syntax should give you the config node, then you can call functions on it like you would normally do. See the node-red-web-nodes repository, it has plenty examples of (core) nodes that do this.
For example the instagram node does this: https://github.com/node-red/node-red-web-nodes/blob/master/instagram/instagram.js