Put this below code in `List A` event receiver

**Code:**

    public override void ItemAdded(SPItemEventProperties properties)
        {
            base.ItemAdded(properties);
    
            using (SPSite site = new SPSite("http://MyWeb/sites/mysite/"))
            {
                using (SPWeb web = site.OpenWeb())
                {                
    				SPList desList = web.Lists.TryGetList("B");
    				if (desList != null)
    				{
    					SPListItem desItem = desList.Items.Add();
    					foreach (SPField field in properties.item.Fields)
    					{
    						if (!field.ReadOnlyField && !field.Hidden && field.InternalName != "Attachments")
    						{
    							if (desItem.Fields.ContainsField(field.InternalName))
    							{
    								desItem[field.InternalName] = sourceItem[field.InternalName];
    
    							}
    
    						}
    					}
    					desItem.Update();
    				}
                }
            }
    
        }

This method will copy the list item from `List A` to `List B` after you add list item to List A.

Hope this will help you!