To make a user an administrator of an Azure subscription, assign them the Owner role at the subscription scope.
The Owner role gives the user full access to all resources in the subscription, including the permission to grant access to others.
The steps are the same as any other role assignment.
Refer to https://www.edureka.co/community/61168/how-do-assign-role-to-particular-user-using-the-azure-portal?
You just have to select the Owner role, in the Role drop-down list.
Hope this helps!