Bypass sanitization with FieldManager's TextArea field which will allows to add HTML in metabox/metafield

functions.php

<?php
/**
 * Sometime we need to take input from user with Field Manager's TextArea field, FM is using sanitize_text_field](https://api.fieldmanager.org/source-class-Fieldmanager_Field.html#123)
 * So below is the code to bypass or use your own validation logic.
 */

$fm = new \Fieldmanager_TextArea(
	array(
		'name'     => '_vk_videoembed_manual',
		'sanitize' => 'validate_embed_field', // Link your own validation function here.
	)
);
$fm->add_meta_box( __( 'Embed Code(Overrides Above `Video URL`):', 'slang' ), array( 'post' ), 'side', 'high' );

/**
 * Bypass string validation/or add your own validation for embed field.
 *
 * @param string $value Field value.
 *
 * @return string Field value.
 */
function validate_embed_field( $value ) {

	// You can add your own validation here to validate $value of the field.

	return $value;
}