Specify custom rank expression for AWS Cloudsearch

Question

I am using AWS Cloudsearch and have a field called Auth Level. It has the following values. I mean, each of the following auth_level is for one row.

"auth_level": "[ \"AUTHORISED\" ]", 
"auth_level": "[ \"AUTHORIZED\" , \"GOLD\" ]",
"auth_level": "[ \"AUTHORIZED\" , \"PLATINUM\" ]", 
"auth_level": "[ \"AUTHORIZED\" , \"SILVER\" ]"

I want to sort the results based on the auth_level value. The sorting logic should be PLATINUM first, followed by GOLD, SILVER, and AUTHORISED. I tried to add the following expression to the expressions section for using it with the sort option.

if(
  contains('PLATINUM', auth_level),
  4,
  if(
    contains('GOLD', auth_level),
    3,
    if(
      contains('SILVER', auth_level),
      2,
      1
    )
  )
)

So my query will be like q=*&sort=my_rank_expression desc, but I cannot add this as it says invalid configuration. How can I specify these custom sort criteria for AWS Cloud search?