You can withdraw your cards to OpenSea on our web application by visiting the card detail page. Visit your collection library, select the collection your card is in, open the card detail view page and then click 'withdraw to Immutable X' on the right side of the page. Once you withdraw the item to Immutable X you can move it to Ethereum and then list it on OpenSea.
Immutable X and Opensea have announced integration support that is currently in development. This will allow you to post items to OpenSea simply by listing them for sale in the Hro platform. We will keep our community posted as we get more information about timing for this feature.