If parking qualifies for tax exempt status in your state, you will only be able to make a Pay Later reservation. You will then need to check out with a cashier and provide your tax-exempt form. The cashier will then apply your tax exemption to your stay.
You will need to show your tax-exempt form at checkout with the cashier for every stay that should be tax-exempt. You will not be able to save your tax exempt form to your Spot Club account.