<?php
/*
Plugin Name: Demo Plugin
Version: 1.0
*/

do_action('basic_action_demo');
?>

add_action('basic_action_demo','action_demo');

function action_demo()
{
    echo "I am inside";
}

<?php
/*
Plugin Name: Demo Plugin
Version: 1.0
*/

add_action('basic_action_demo','action_demo');
do_action('basic_action_demo');

function action_demo()
{
    echo "I am Inside";
    die;
}
?>

<?php
/*
Plugin Name: Demo Plugin
Version: 1.0
*/

add_action( 'after_setup_theme', function() {
    do_action( 'basic_action_demo' );
} );
?>

<?php
/*
Plugin Name: Demo Plugin
Version: 1.0
*/

function output_my_form () {
    echo "I'm a form";
    // do your action here
    do_action( 'basic_action_demo' );
}
?>

if ( function_exists( 'output_my_form' ) ) {
    output_my_form();
}