Automatically apply custom field value in post creation WordPress

timDesain Nanang answers:

try:
put this code in the ifttt's body field (see screenshot):
[source_link] {{PostUrl}} [/source_link]


put this code in the theme's functions.php

//saving post meta
add_action( 'save_post', 'timd_save_meta' );
function timd_save_meta( $post_id ) {
$get_post = get_post( $post_id );
$post_content = $get_post->post_content;

//based on get_shortcode_regex function
$pattern = '\[(\[?)(source_link)(?![\w-])([^\]\/]*(?:\/(?!\])[^\]\/]*)*?)(?:(\/)\]|\](?:([^\[]*+(?:\[(?!\/\2\])[^\[]*+)*+)\[\/\2\])?)(\]?)';
preg_match('/'.$pattern.'/s', $post_content, $matches);

if (is_array($matches) && $matches[2] == 'source_link') {
$meta_value = esc_url(trim($matches[5]));
update_post_meta( $post_id, 'source_link', sanitize_text_field( $meta_value ) );
}

}

//hide shortcode in frontend
add_shortcode('source_link', 'timd_source_link');
function timd_source_link( $atts ){
return '';
}