Getting back to the original problem... A couple of thoughts:
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Firstly, you should have a POI (point of interest) category for Ferries on the TomTom.
You should be able to go:
1. Navigate to...
2. Point of Interest
3. POI in City
4. You then have to tap the little UK flag in the top right corner and select "France" to change the city search to France
5. Type in "Calais"
6. Tap the arrow, bottom right
7. In the POI category list, either use the "down arrow" and scroll down (or use "search" and type "F") until you find the "Ferry Terminals" category.
8. "Terminal car ferry" should be one of the places listed (along with the Eurotunnel and other places within 30 miles or so) within a mile of Calais centre.
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Secondly, if you wanted to use Lat/Long coordinates, I'm not sure whether you're saying you can't work out how to enter the co ordinates on your ONE, or if you just can't find the right coordinates to enter.
Some ONE models may not be able to use Lat/Long co-ordinates, but if yours can, try tapping:
Navigate to...
Right arrow
Latitude/Longitude
... then entering:
N50.96777, W1.86894 - that's the service road to the ticket office, or
N50.96675, W1.86910 - that's the entrance if you already have tickets.
Both are off the same access road.
(I found those from Google maps and StreetView)
Edit: OOPS... see Anchor's correction below