Since domain www.styledumonde.com registered at www.mydomain.com, you will need to log into the panel at www.mydomain.com There should be an option for you to modify its DNS record. You will have to check with them.
For your reference, if you want to point a domain registered at DH to elsewhere, you can go to DH panel --> Domains --> Manage Domains --> Web Hosting --> Edit. You can check whether there is a similar feature at www.mydomain.com
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