Currently, the SSLParameters javadoc has an example String for coding purposes:
https://docs.oracle.com/en/java/javase/16/docs/api/java.base/javax/net/ssl/SSLParameters.html

// 0x00-0xFF: 1 byte
     String rfc7301Grease8F = "\008F\008F";

The Unicode encoding is being lost. I think either of these values in the src code should work:

String rfc7301Grease8F = "\\u008F\\u008F";
String rfc7301Grease8F = "\u005cu008F\u005cu008F";