Need help with the help bar

I have a few questions about the nav-bar.

1. I would like for my nav links to be a clickable URL. When you hover I would also like a drop-down menu to appear which I have done by using this CSS .
.dropdown:hover > .dropdown-menu {
  display: block;
}
My problem is the navlink is not working the URL will appear in bottom left corner but not responding when I click.
. Can you please explain why?

2. I would like for the drop-down menu to display below the bottom border of the navbar. I know I can do this with margin-bottom is there a better way to do this? My only problem my header shrinks on scroll. For some reason i can shrink the header logo but it's not allowing me to change the margin-top here is my code.

--------------------------------------------------------------------------------------------------------------------------------
// When the user scrolls down 50px from the top of the document, resize the header's font size
window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 50 || document.documentElement.scrollTop > 50) {
    /*THIS DOSENT WORK*/document.getElementByClassName("dropdown-menu").style.margin-top = "20px"; 
    document.getElementById("header").style.width = "200px";



  } else {
    document.getElementById("header").style.width = "300px";

     /*THIS DOSENT WORK*/ document.getElementByClassName("dropdown-menu").style.margin-top = "35px";
  }
}

-----------------------------------------------------------------------------------------------------------------------------------------------------     
Here is my whole nav bar code:
<nav class="navbar navbar-light navbar-expand-lg fixed-top">
    <div class="container"><img src="Lytrod_Corporate_Logo-1.png" /><button data-toggle="collapse" data-target="#navcol-1" class="navbar-toggler"><span>Toggle navigation</span><span></span></button>
        <div class="collapse navbar-collapse d-xl-flex justify-content-xl-center"
            id="navcol-1">
            <ul class="nav navbar-nav">
                <li class="nav-item dropdown"><a href="blank">PRODUCTS</a>
                    <ul role="menu" class="dropdown-menu menu-drop menu-drop-small" >
                    <li><a href="">blank</a></li>
                    <li> <a href="">blank</a></li>
                    <li>  <a href="">blank</a></li>
                    <li> <a href="">blank</a></li>
                    <li> <a href="">blank</a></li>
                    <li>  <a href="">blank</a></li>
                        <li><a href="">blank</a></li>
                        <li><a href="">blank</a></li>

                    </ul>
                </li>
                <li class="nav-item dropdown" id = "drop"> <a href="blank">SUPPORT</a>
                    <div role="menu" class="dropdown-menu">
                      <a href="">blank</a>
                      <a href="">blank</a>
                      <a href="">blank</a>   
                    </div>
                </li>
                <li class="nav-item dropdown">
                    <a href="">DEMO CENTER</a>
                    <div role="menu" class="dropdown-menu">
                        <a href="blank">blank</a>
                        <a href="blank">blank</a>
                        <a href="blank"></a>
                         <a href="blank">blank</a>
                    </div>
                </li>
                <li class="nav-item dropdown"><a href="https://www.lytrod.com/company/">COMPANY</a>
                    <div role="menu" class="dropdown-menu">
                        <a href="blank">blank</a>

                        <a href="blank">blank</a>

                        <a href="blank">blank</a>

                        <a href="blank">blank</a>
                    </div>
                </li>
                <li class="nav-item dropdown"><a href="">STORE</a>

                </li>
            </ul>
        </div>
    </div>
</nav>

... a few questions? ...

tl;dr

I like learning from video and after watching this, maybe the navbars and how they work will make more sense to you.

http://youtu.be/9cKsq14Kfsw

You can pick the video up at 7 minutes into it without missing anything really important.

I hope this helps.

... After watching, go through some of the tutorials included with Bootstrap Studio (BSS). The nav-bar is mostly drag-n'-drop, that simple. There isn't really all that much to do, really.