You could do some custom code like this:
<form action="https://www.e-junkie.com/ecom/gb.php?i=XXXXXX&c=cart&cl=YYYYY&ejc=2" method="POST" target="ejejc">
Quantity:<input type="text" name="quantity" size="3" />
<input type="image" src="http://www.e-junkie.com/ej/ejadd_to_cart.gif" border="0" alt="Add to Cart" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this.parentNode);"/></form>
Basically, just take the href= URL from the original Add to Cart button code we provide, and insert that URL as the form action= URL (replacing the example URL above).
If your product happens to be configured with Variations or Variants, then our standard button code is already provided as a form like the above, so you'd just need to add this tag inside the form:
<input type="text" name="quantity" size="3" />
