wp_create_term( string $tag_name, string $taxonomy = 'post_tag' )

Source

File: wp-admin/includes/taxonomy.php

function wp_create_term( $tag_name, $taxonomy = 'post_tag' ) {
	$id = term_exists( $tag_name, $taxonomy );
	if ( $id ) {
		return $id;
	}

	return wp_insert_term( $tag_name, $taxonomy );
}