How to add apply_filter for a class property in plugin

Your problem has nothing to do with apply_filters, it’s a basic PHP mistake:

            public function init (){
                $filter_fields = apply_filters('tbg_reg_fields', $this->fields);
            }

Here, you create a brand new variable in the init method scope named $filter_fields, then never use it.

$this->filter_fields and $filter_fields are not the same.

Additionally, because you used a filter and stored the fully formed HTML in advance, it’s impossible to late escape, leading to security issues.

So instead, replace this:

                    <h4>Register as a New Store Owner</h4>
                        <form method="post" class="tbg_reg_form" action="' . admin_url( 'admin-ajax.php' ) . '">
                            <?php echo $form_fields; ?>
                            <input type="hidden" name="action" value="tbg_form_submit">
                            <input type="submit" value="Start Selling Now">
                        </form>

With a do_action, like this:

                    <h4>Register as a New Store Owner</h4>
                        <form method="post" class="tbg_reg_form" action="' . admin_url( 'admin-ajax.php' ) . '">
                            <?php do_action( 'tbg_reg_fields', $this->fields ); ?>
                            <input type="hidden" name="action" value="tbg_form_submit">
                            <input type="submit" value="Start Selling Now">
                        </form>

That way:

1. you can output the HTML directly
2. the action only runs when the code is being generated, unlike the original which runs even when there are no shortcodes on the page
3. You can escape the fields in the functions that output them
4. The call to init method and the filter_fields variable can be completely eliminated

Further general code quality notes:

• You’re using array( $this, '.... use PHP 5.3+ short array syntax instead [ $this, '... instead
• It’s impossible to load class-registration-form.php without creating that class, making it impossible to create unit tests
• There’s no reason for this to be a class, the entire thing could be simplified to a function named create_reg_form, moving the fields array into that function, and calling add_shortcode on the init hook. The class provides nothing, it isn’t object oriented, and makes the code more complex, not less, and you only ever have one instance, so it’s a waste of memory to create an object
• admin-ajax.php is for really old AJAX handlers! Stop abusing it as a form handler! Leave the action blank and then check $_POST on init to check if your form got submitted and handle it there. Don’t abuse admin-ajax.php for things it was never intended for. if ( !empty( $_POST['action'] ) && $_POST['action'] === 'tbg_form_submit' ) on the init action is more than enough