server-tools/auth_supplier/views/auth_supplier_view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <template id="auth_supplier.fields"
              inherit_id="auth_signup.fields"
              name="Auth Signup/ResetPassword form fields">
        <xpath expr="//div[@class='form-group field-login']" position="before">
            <div class="form-group field-type">
                <label for="account_type" class="control-label">Account Type</label>
                <select class="form-control" name="account_type">
                    <option value="customer" selected="selected">Customer</option>
                    <option value="supplier">Supplier</option>
                 </select>
            </div>
        </xpath>
    </template>

</odoo>