I'm not sure how you design that dropdown.
If it is an enumeration, go to domain model and set the default value for that attribute. It is at the bottom of the attribute screen.
If it is a reference to another entity, you will have to set the reference to the object before passing it to the page.
Did you mean that you want 1 of the dropdown values to be preselected?
then you should use a datasource microflow for this Dropdown and set that to the context entity and its association. That way 1 element will always be pre-selected. Then once you change the selection it will be set itself to the association.