Amir, i would wonder about that friend too.
If your problem is a high mineral salt content in your water, you should be wary of a friend who tells you to add more salt to fix your problem?
If you really do mean "soap", as in Ivory brand, it will produce a lot of foam which might look nice in your sink and be gentle on your hands but make a mess in your cooker. Foam is a very poor heat conductor too so you might damage your Anova's element and impeller. Bubbles don't clean. They just look nice, - but belong on beer, not in cleaning solutions.
And if you meant a detergent, they are not much better but the best ones contain expensive anti-foaming compounds to improve their action. However they are compounded to remove food soil, starches, and fats; not so much minerals.
If you resist Allyssa's cleaning advice you probably could slightly acidulate your cooking water with a little vinegar, but of course your home will smell like a salad whenever you cook.
Got two spare minutes?
If you insist on using an alternative to Alyssa's advice, just put about 4-inches depth of straight 5% white vinegar in a clean glass jar. After every cook, dip your Anova into the jar of vinegar and hold it there for a minute. Half that time would probably be enough in water of less than 10-grain hardness. Remove the Anova and thoroughly rinse inside it with cold running water. Put a lid on the jar of vinegar, label it it, and save it for subsequent cooks. The vinegar will be good until it isn't, as in very cloudy.