subscription-form.php

<?php
// Add Shortcode
function subform() {

    // Start output buffering
    ob_start();

    // Get the current user's info
    $current_user = wp_get_current_user();
    $user_display_name = !empty( $current_user->user_firstname ) ? $current_user->user_firstname : $current_user->display_name;

    // Get the blog name
    $blog_name = get_bloginfo( 'name' );

    // Display the welcome message and subscription form
    echo '<p>Hey ' . esc_html( $user_display_name ) . ', welcome to ' . esc_html( $blog_name ) . '! You can subscribe to our newsletter here:</p>';

    ?>
    <form action="/thank-you">
        <label for="email">Enter your email:</label>
        <input type="email" id="email" name="email">
        <input type="submit" value="Submit">
    </form>
    <?php
}
add_shortcode( 'subscriptionform', 'subform' );