Hi there!
Just to verify, what is the name of the file that contains this function?
If you would like to make an external API using http functions , it must be contained within http-functions.js.
You can create it by clicking on Public & Backend > Backend > Expose site API
If this is resolved, verify that the path you are making the GET request to is correct. Assuming you have connected your own domain, the path should be (based on your function get_myAPI):
https://www.mysite.com/_functions/myAPI