How do I add custom_meta_box_id's value?

Question

I have added custom meta boxes in post and I want to put Tag slug name in the meta box text field, and want to display the text field's texts (Tag slug name) like following this.

<?php 
$my_query = new WP_Query( 'showposts=4&tag=' . $CUSTOM_META_BOX_ID_VALUE ); 

while ( $my_query->have_posts( )) : $my_query->the_post();  

//Post stuff.....  

endwhile; 
?>

Please can anybody help me?

I mean I'm trying to display related/recent posts by some specific tags for some specific posts. So I am trying it with Option Tree custom meta box code. Meta box register code is simply like this-

$my_meta_box = array(
'id'          => 'tsi_mp3_meta_box',
'title'       => __( 'Music Options', 'tsi' ),
'desc'        => '',
'pages'       => array( 'post' ),
'context'     => 'normal',
'priority'    => 'high',
'fields'      => array( 
 array(
'label'       => __( 'Tag Name <code>This is necessary for display Albums
contents & related posts</code>', 'tsi' ),
'id'          => 'tsitg',
'type'        => 'text',
'desc'        => __( 'Type the TAG name you added for this post.', 'tsi' )
  ),
)
);

And it appears like this in single post -

bangerz_miley_cyrus is the tag slug name of the post. And I want to display related posts under this tag in certain posts. So I want to add the value of 'tsitg' as tag slug name in WP_Query object something like this-

$my_query = new WP_Query( 'showposts=4&tag=' .$tsitg ); 
while ( $my_query->have_posts( )) : $my_query->the_post();  

So here I'm asking for help...