Add a custom class to the body tag using custom fields

You want to use body_class filter.

function prefix_add_body_class( $classes ) {
    global $post;

    // good to check
    if( ! is_single() || 'post' !== get_post_type() ) {
        return $classes;
    }

    $btag = get_post_meta( $post->ID, 'tagbody', true );

    if ( empty( $btag ) ) {
        return $classes;
    }

    $classes[] = $btag;

    return $classes;
}
add_filter( 'body_class', 'prefix_add_body_class' )

EDIT:

You wrote me in the comments that the theme is your own theme. So you don’t need to do it via a filter. Just do it right in your theme.

Edit the header.php file like this.

// some code above
$classes = array();

if( is_single() && 'post' === get_post_type() ) {
    $btag = get_post_meta( $post->ID, 'tagbody', true );

    if( ! empty( $btag ) ) {
        $classes[] = $btag;
    }
}
?>
<body <?php body_class( $classes ); ?>>
<?php
// some code below

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