That is just another way to identify a field. Fnm = field name similar to how fid = field id. They can usually be used interchangeably, I just copied the URL alternative QuickBase gave.
Also I think this help doc might be good to read. If you do the URL alternative I mentioned previously you will need to make it a GET request, one of the things it mentions.
https://help.quickbase.com/api-guide/understandreference.html#sample_textAnother option to do what you are wanting would be using some kind of intermediary like Workato or Zapier or something else that can send a webhook. That might be easier to do but that will depend no how comfortable you are with coding.