To login to Zen-Cart system either as a customer or a store Admin, Zen-Cart restricts access to a form that contains the securityToken value. This is a must. So if you need to build a log-in form in the outside of Zen-Cart framework, you must find a way to include this securityToken before you submit to data to the Zen-Cart login process.
Beside the log-in mechanism, Zen-Cart has the cart functionality that require the securityToken also. Here’s all the data values that you need to submit to “add to cart” process.

<input name="securityToken" type="hidden" value="blahblahblah...64d7d0acdbe575ca9af8e62822568f92" /> 
<input maxlength="6" name="cart_quantity" size="4" type="text" value="1" /> 
<input name="products_id" type="hidden" value="21" />