If you have eCart installed you can add the eCart > general > set session value server behavior to set the session variable for you.
You need to apply this to the page that your form is posting to with the action in the form tag. When you apply this you will set the trigger, like before page load, then for the value use the lightning bolt to select the select list.
If you are adding it to the same page you will have the form bindings available to you, if you are applying it to a page that does not have the form on it you will need to go to Bindings > Form Data to select the page that has the form on it to get the bindings available to you.