functions.php

<?php

// call this method inside the method that shows the metabox contents
function prefix_upload_inputs() {
    global $post;
    
    wp_enqueue_media();
    $scripts = "
        <script type='text/javascript'>
            jQuery(document).ready(function($){
                var PF_Upload = {
                    button: '',
                    ready: function() {
                        PF_Upload.button = $('.jMediaUploader');
                        PF_Upload.binds();
                    },
                    binds: function() {
                        PF_Upload.button.click(PF_Upload.do_upload);
                    },
                    do_upload: function(e) {
                        e.preventDefault();
                        var self = $(this),
                            input = self.attr('id').replace('_button', ''),
                            send_attachment_old = wp.media.editor.send.attachment;

                        wp.media.editor.send.attachment = function(props, attachment) {
							var chosen_size = props.size;
                            $('#' + input).val(attachment.sizes[chosen_size].id);
                            wp.media.editor.send.attachment = send_attachment_old;
                        };

                        wp.media.editor.open(self);
                    }
                };

                PF_Upload.ready();
            });
        </script>

    ";
    
    $name = ''; // name of setting
    $value = get_post_meta( $post->ID, $name, true );
    $input_text = "<input type='text' name='{$name}' id='{$name}' value='{$value}' />";
    $input_button = "<button name='{$name}_button' id='{$name}_button' class='button button-secondary jMediaUploader'>" . __( 'Upload' ) . '</button>';
    
    echo $input_text . $input_button;
    
}