How do I find the account linked to an Office Home & Student 2013 key?

Hello everyone,

Microsoft's after-sales service redirected me here because they no longer provide updates for this type of product, nor even security support.

I have two Microsoft accounts. However, when I try to reconnect my key to one of them, it tells me the key is already linked to another Microsoft account. But which one?! How can I find that account or regain ownership of my Office key ?

Thanks for your help.