Am I right in thinking that the solution you went with from the other thread was to have a reservation post type and a one-to-many relationship between ports and reservations, and another one-to-many relationship betweem ships and reservations?
And so the form you are talking about here is a form to publish reservation posts, which is shown on the port page, and which has the port pre-selected and from which a ship can be chosen. (I assume the arrival date is a field belonging to the reservation post.)
To auto-populate the post title there are two ways you might do this.
One would be to add JavaScript to your form. The post title field would be included (it is needed) but hidden, and you could add on-change event handlers to the arrival date field and the ship selector which assembles the title from the 3 field components, and then the post title will be handled normally as the form is submitted.
In your case you would have the ship and port IDs available in the $_POST object and you would need to get those post objects (with get_post) to access their titles.
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