Slowing block

This is probably possible in MCreator using procedures, however, if you need/want to code here you go;

   /**
    * Called when the given entity walks on this Block
    */
   public void onEntityWalk(World worldIn, BlockPos pos, Entity entityIn) {
      double d0 = Math.abs(entityIn.getMotion().y);
      if (d0 < 0.1D && !entityIn.isSteppingCarefully()) {
         double d1 = 0.4D + d0 * 0.2D;
         entityIn.setMotion(entityIn.getMotion().mul(d1, 1.0D, d1));
      }

      super.onEntityWalk(worldIn, pos, entityIn);
   }

From SlimeBlock.java.

Oh thanks mate, how did you find the .java from a block/item etc... ?

IDE

But for coding, I don't know where to start, can I use mcreator to use copy paste this code ?

Yes, you can use MCreator. Just put it inside the last bracket, }.