Technical contacts of public cloud application can be managed by the customer (Self Service) in the Mendix Cloud Portal.
Cloud Portal (home.mendix.com): Project > Project Settings (Gear icon) > Node security > Select desired technical contact (must be a project member)
Call the support desk. They can do this for you.
Logged in is a Technical Contact, go to Environments – Permissions and click on the well hidden hyperlink under the name you want to make technical contact :