find custom post type post by searching its custom field with my string

You can find the post using get_posts() and the meta query (in my example, I used meta_key and meta_value) like so:

See the function reference for more details on the parameters (and other examples that might help you), but the 'fields' => 'ids' means that the function will return only the post IDs and not full post object/data.

<?php
// Find a "series" post with the meta originaltitle set to a specific value.
$ids = get_posts( array(
    'post_type'      => 'series',
    'post_status'    => 'publish',
    'posts_per_page' => 1,
    'meta_key'       => 'originaltitle',
    'meta_value'     => 'put value here',
    'fields'         => 'ids',
) );

$post_id = array_shift( $ids );

// Or you can do something like:
/*
if ( ! empty( $ids ) ) {
    $post_id = $ids[0];

    // run your code
}
*/