Vkumudini -- I make a powdered soap laundry soap, and it works very well for me even when washing in cold water, but there was a learning curve I went through for my first few batches.
You can put powdered soap directly into the washer tub or into the soap dispenser, whichever works best for you. I add the soap either way and get decent results, but I gravitate toward adding it to the tub. If you want to add the soap to the tub too, put it in the bottom so it gets mixed with water and get started dissolving as soon as possible. If you have a HE washer like I do, this is important, because there's not much water in the tub -- soap sprinkled on top of the clothes may not get a chance to dissolve right away.
There are three things to remember when using any powdered soap in a washing machine --
(1) Make the soap as water soluble as possible by using mostly coconut oil (or PKO). I use 70% CO and 30% lard, but many use 100% CO. You probably already know this, but I'll include this for the benefit of others who might not.
(2) Also keep the powder as fine as possible. You don't want granules the size of "cracked pepper", you want a powder closer to fine grained table salt. Do this by powdering the soap with a food processor within 24 hours of making the soap. The point is to not let the soap dry too much after it's made, or it will be too firm and hard to physically shatter into powder that's fine enough to dissolve fast. This is especially true of a high CO soap.
(3) Last, if you see particles of soap remaining on your clothes after washing, try using warmer water for washing until you use up this batch of soap or dissolve the soap in warm water before use. Warmer water in the wash will increase the solubility of the soap so it is more likely to fully dissolve. And if (3) happens, rethink your strategy for (1) and (2) so your next batch of laundry soap will work better.