A way to programmatically add a new block to a post using `parse_blocks()` and `serialize_blocks()` in WordPress. See https://www.facebook.com/groups/advancedwp/posts/7007114406017380/?__cft__[0]=AZWLn3Sa6BYQ9n2qmGTu1j2bPNBnUYdVBpYMEt8wfhaNSResEI_d-TICOX8GuVed1wDNlNq3Dxjh7iSxJnDNPVwgCvFwyGoM3HecqseR2cXeGhda3EfMslTHrfeQiFPwbG801Tl3scboksvZMDKsy0O…

parse_and_serialize_blocks.php

<?php
function rh_filter_the_content( $the_content = '' ) {
	$blocks    = parse_blocks( $the_content );
	$new_block = array(
		'blockName'    => 'core/paragraph',
		'attrs'        => array(),
		'innerBlocks'  => array(),
		'innerHTML'    => '<p>Hello World</p>',
		'innerContent' => array(
			'<p>Hello World</p>',
		),
	);
	$blocks[]  = $new_block;

	return serialize_blocks( $blocks );
}
add_filter( 'the_content', 'rh_filter_the_content', 5 );