I just tried my "test account" which is not associated with a MRN. It appears I can make donations just like a member. The one difference is that one needs to be careful to select a "missionary in other ward or branch". Otherwise it goes to "Ward Missionary Fund USA - Salt Lake City (COB)".
Everyone outside your ward will need the unit number if your ward.
Note that you'll either have to coordinate with family and/or ask the ward finance clerk for updates to the account. Extra care will need to be taken toward the end as there is no way for you to see the balance and all donations are non-refundable.
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