In Admin Setting: Use Installation ID for the field Merchant ID Field
For signature field add the following value
instId:amount:currency:cartId:postcode
![](https://www.huntbee.com/resources/wp-content/uploads/2020/05/worldpay-merchant-settings.jpg)
Reference: Testing Guide from Worldpay
http://support.worldpay.com/support/kb/bg/testandgolive/tgl0000.html