https://www.epo.org/en/node/online-only-1

Is it obligatory to indicate in my refund instructions a deposit account held by a party to the proceedings, e.g. the European representative or the applicant?

No. According to point 15.1 ADA, you can also indicate a deposit account held by a third party. However, the refund instructions themselves must always be filed by a party to the proceedings because third parties are not allowed to act in proceedings before the EPO. Refund instructions filed with the EPO by a third party will therefore be ignored.