Perch Shop, pass in variant value automatically

Hi,

We're using perch shop for memberships and ticket sales on our site. For memberships, there's a new request that we tag people who come from a certain affiliated web site as coming from that site to sign up. So what I've done is created a new join url to receive visitors from that site and created 2 variants for each membership level product...

And what I would like to do is feed in that oter-site variant (without user interaction) to the product add to cart.

this is my current add to cart form:

<perch:form id="add_to_cart" app="perch_shop" action="/cart">
    <perch:input id="product" type="hidden" env-autofill="false" value="<perch:shop id="productID" type="hidden" env-autofill="false" />" />

    <perch:input type="submit" class="pricing--action" value="Add to cart" />
</perch:form>

Here's the record for the membership level with the variants:

_variant_opts: TownGreen2025|64,GMF|65

Array
(
    [0] => Array
        (
            [title] => TownGreen2025
            [precendence] => 1
            [optionID] => 5
            [optionTitle] => TownGreen2025
            [optionPrecendence] => 1
            [optionDynamicFields] => {"title":"TownGreen2025","precendence":"1"}
            [optionCreated] => 2016-12-20 16:56:36
            [optionUpdated] => 2016-12-20 11:57:15
            [optionDeleted] => 
            [perch_title] => TownGreen2025
            [perch_precendence] => 1
            [options] => Array
                (
                    [0] => Array
                        (
                            [id] => 34
                            [title] => TownGreen2025
                            [skucode] => TWNGRN
                        )

                    [1] => Array
                        (
                            [id] => 35
                            [title] => GMF
                            [skucode] => GMF
                        )

                )

            [productvalues] => Array
                (
                    [0] => Array
                        (
                            [prodoptID] => 49
                            [optionID] => 5
                            [valueID] => 34
                            [optionTitle] => TownGreen2025
                            [valueTitle] => TownGreen2025
                            [valueSKUCode] => TWNGRN
                            [valueOrder] => 1
                            [valueDynamicFields] => {}
                            [valueCreated] => 2016-12-20 16:56:36
                            [valueUpdated] => 2016-12-20 11:57:15
                            [valueDeleted] => 
                        )

                    [1] => Array
                        (
                            [prodoptID] => 50
                            [optionID] => 5
                            [valueID] => 35
                            [optionTitle] => TownGreen2025
                            [valueTitle] => GMF
                            [valueSKUCode] => GMF
                            [valueOrder] => 2
                            [valueDynamicFields] => {}
                            [valueCreated] => 2016-12-20 16:57:15
                            [valueUpdated] => 2016-12-20 11:57:15
                            [valueDeleted] => 
                        )

                )

        )

)

Can I pass in the Towngreen2025 variant with a perch:input hidden field? What would it look like?

Thanks, Monty