Run the "Basic Lemmy API login script" (see here) first to get value for 'jwt'

#!/bin/bash

# Basic get unread replies script for Lemmy API

# CHANGE THESE VALUES
my_instance=""			# e.g. https://feddit.nl
my_username=""			# e.g. freamon
my_password=""			# e.g. hunter2

jwt=""			# run basic login script to get this

########################################################

# Lemmy API version
API="api/v3"

########################################################

# Turn off history substitution (avoid errors with ! usage)
set +H

########################################################

# Get inbox messages
get_unread_replies() {
	end_point="user/replies"
	www_data="auth=$jwt&unread_only=true"

	url="$my_instance/$API/$end_point?$www_data"

	curl "$url"
}

get_unread_replies

Personally, I'd use the 'jq' program to de-serialize the reply, e.g.

get_unread_replies.sh | jq '.replies[].comment.content'

My next plan is to use a script like this with a notification service like PUSHOVER, so it can ding my phone if I've an unread message

For each API method, whether it's GET or POST and end_point can be found here, and then each method has a click-through to see what parameters it can take.