POST with App Script for Google Sheets

Hello there,

I am working on my own personal site.
I have recently updated the contact form when the CONTACT button is clicked. A modal overlay opens up and displays the form, with a scrollbar.

I've now been trying to link the form's input, using Googles App Script API features, to a Google Sheet.

I guess I should have a better understanding of

POST

POST

and

FETCH

FETCH

, though it looked simple enough,
yet I'm not getting any results.

Google App Script: app_script.png (Code.gs)

Code Pen: https://codepen.io/John-Roby/pen/bGOXjxL
I'm not sure this works in Code Pen and is just for sharing my