If you live in the upper Midwest like I do, this Artic Invasion and snow has left rock salt inoperable as a de-icer. It worked with the first wet snow before the sub-zero Fahrenheit temps arrived, but when they did, it was useless. The public works department, both city and county, did put down A LOT of salt in an effort to make well-travel streets/roads/highways and intersections somewhat safe, but have since resorted to sand and small-grained gravel at side street intersections to make them safer. Some of the lesser traveled road are glare ice from the vehicle travel compacting that snow into rock-hard glare ice.
All I can say is between the salt and dirt, there are a lot of VERY dirty vehicles in Wisconsin and they all have ermine-white salt residue on them and the bottom rails and lower quarter panels have rock-hard, sand-embedded frozen slush chunks on them. Even just travelling clear streets, the salt dust just hangs in the air like a fog and even a rarely seen clean vehicle that sticks out like a soar thumb in traffic will become coated in this salt dust.
As far as cleaning vehicles and "pre-treating" them prior to a High-Pressure (HP) wash in an effort of not having to physically touch surface and risk inducing swirls and scratches, well, I find that kind of a pipe-dream fantasy and having a truely "clean" paint surface. I find that physical mechanical touching by an "appropriate" wash media (microfiber noodle mitt for me) is still the best way to clean a salt and dirt encrusted surface. If a salt binder, such as beet juice, has been used to make the salt stick to the road surface, this gooey concoction cannot be removed by any other method completely that with a physical mechanical touching to wipe off such Road Traffic Film (RTF). I know this too well after going through touch-less tunnel washes and then drying my vehicle with PFM microfiber drying towels. I do this immediately after going through the wash to mitigate freezing of the water that the end-of-the tunnel blower does not remove, especially door jams and trunk jams. I am willing to live with drying towel-induced swirls just to have the salt and RTF removed completely.
There are some pre-soak soaps out there that come to mind; like 3D`s Super Pre-Soak, or Blackfires Foam Soap, but I have no experience with an HP washer , foam guns/lances and pre-soak soaps, and I am sure that IS what the Original Poster is asking about: "What Pre-soak soap works best at removing salt RTF?" (So what you have told us Captain Obvious is basically nothin`!)
For what it is worth, I have been using a mix of Rain-X Heavy Duty Truck-&-SUV Wash soap and Turtle Wax`s M.A.X.-Power Car Wash "liquid" in about 2 ounces of the Rain-X HD with about 4 ounces of the TW MAX-Power per gallon (usually only make about 2.5 gallons for a wash soap bucket) as economic wash soap for my winter weather-appropriate (above 32°F) two-bucket washes. Yes, I do the panels and jams first, then the wheel wells and rims/tires last with the same soap bucket (different wash media for the wells and rims/tires, though) As stated, both Rain-X soap and Turtle Wax liquid wash are inexpensive (AKA cheap, like me), work very well at removing salt RTF, and are easy to find at over-the-counter big-box stores or auto parts stores.