How to add Delete DDL statement through variable in SSDT

Question

I want to delete partitions for specific date via delete DDL statements using variables in SSDT.

Currently getting error as "Invalid input"

I tried with direct input option also, but it doesn't work.

Below code works fine when static date is passed as directinput:

{
  "delete" : {
   "object" : {
          "database": "TestDatabase",
          "Table" : "Delta",
          "partition" : "Delta 20200501" 
     }
   }
}

I tried with passing variable to the date, but it doesn't work:

{
      "delete" : {
       "object" : {
              "database": "TestDatabase",
              "Table" : "Delta",
              "partition" : "Delta " + REPLACE(@[USER::ExpiryDate],"-",""), + " \"
         }
       }
    }

what is the exact delete syntax for deleting partition and how can I use a variable instead of static date?

Thank you