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I will assume that will enter post ids into a posts custom field, comma separated, eg: 11,13,34,54 OR 11, 13, 34, 54.

Then all you need to do is get the custom field value for the loaded post, explode the custom field value by comma(,): and, then you'll have a nice array to pass into the include parameter. Here is an example showing the important pieces:

<?php
    $postIds = get_post_meta($post->ID, 'postIds', true); // get custom field value
    $arrayIds = explode(',', $postIds); // explode value into an array of ids
    if(count($arrayIds) <= 1) // if array contains one element or less, there's spaces after comma's, or you only entered one id
    {
        if( strpos($arrayIds[0], ',') !== false )// if the first array value has commas, there were spaces after ids entered
        {
            $arrayIds = array(); // reset array
            $arrayIds = explode(', ', $postIds); // explode ids with space after comma's
        }
        
    }

    $args = array(
        'include' => $arrayIds // pass array of ids into `include` parameter
    );
    ...
?>