Normally, the Discount Code box would only be displayed in the cart if you have any discount configured for "All Cart Items" or for any item the buyer has added to their cart. However, you can remove it by adding this cart customization code to your View Cart code on every page:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#dscnt_cd").remove();
jQuery("input[name='discount_code']").remove();
jQuery("#discount2").remove();
}
If you are already using function EJEJC_config(){} and/or function EJEJC_shown(){}, just add the jQuery lines provided above to your existing function EJEJC_shown(){} section.
Now, to create a link that would apply a discount code to the buyer's cart, you would need to add &discount_code=YOURCODEHERE to an Add to Cart button URL, so clicking the button or following the link URL would apply that discount code to the buyer's cart -- e.g., here's some modified Add to Cart button code:
<a href="https://www.e-junkie.com/ecom/gb.php?c=cart&i=XXXXXX&cl=12103&ejc=2&discountcode=YOURCODEHERE" target="ej_ejc" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this);"><img src="http://www.e-junkie.com/ej/ejadd_to_cart.gif" border="0" alt="Add to Cart"/></a>
You may want to create a dummy product priced at 0.00 for this, so you can direct your selected buyers to a secret page on your site, where they'd click that button to add the dummy item to their cart and claim their discount.