？
On that occasion, should I attach the homepage URL?
Literal
That occasion homepage URL [topic-は] attach should-[copula-なんでしょうか]?
その際 '(on) that occasion' is a formal variant of その時, stiffer and more written. ホムペ is the slang abbreviation of ホームページ 'homepage' (which itself, in Japanese usage, has shifted to mean 'website'). 添付する 'to attach' is a business/technical verb for attaching files or documents. ~べき 'should / ought to' is a formal modal. ~んでしょうか is the hedged polite question ending.