I'd stick with plain water for seedlings. Some growers use a vitamin solution, but you really don't have to, and the risk of messing with newly-popped beaners is not one you want to take unless you do a test run on shwagseed (bring them to 5 nodes at good health and that will tell you if it is okay or not- the test should take 2-3 weeks).
Those Thiamine supplements might have something else in them too- SuperThrive has a lot of Urea and is very 'hot' for seedlings. You can overapply it very easily by mistake!