When you need to convert a string to a UTF8 byte array (a blob) go like this:

orgdata.StringToBase64.Base64ToBlob

Here is an example of calculating a complex token for Svea-pay:

-- as defined in https://checkoutapistage.svea.com/docs/#/getting-started
let orgdata=requestbody+SveaSettingsSingleton.oclSingleton.MerchantSecret+timestamp in
(
  let thehash=SysSingleton.oclSingleton.SHA512ComputeHash(orgdata.StringToBase64.Base64ToBlob) in
  (
    (SveaSettingsSingleton.oclSingleton.MerchantId+':'+SysSingleton.oclSingleton.BitConverterToString( thehash ).Replace('-','')).StringToBase64
  )
)