Expected value of init in AMediaDrm_getKeyRequest for PlayReady

Question

Context:

I want to decrypt EME content of type video/mp4 on Android. I opened a MediaDrm instance using PlayReady UUID. AMediaDrmKeyType is KEY_TYPE_STREAMING.

I am unable to obtain a key request from AMediaDrm_getKeyRequest. I suspect the init data I provide is incorrect, because it's the only parameter I'm not certain about.

The only information I can find is in the library's header and not very specific:

/*
* init container-specific data, its meaning is interpreted based on the mime type
* provided in the mimeType parameter.  It could contain, for example, the content
* ID, key ID or other data obtained from the content metadata that is required in
* generating the key request. init may be null when keyType is KEY_TYPE_RELEASE.
*/

media_status_t AMediaDrm_getKeyRequest(AMediaDrm *, const AMediaDrmScope *scope,
    const uint8_t *init, size_t initSize, const char *mimeType, AMediaDrmKeyType keyType,
    const AMediaDrmKeyValue *optionalParameters, size_t numOptionalParameters,
    const uint8_t **keyRequest, size_t *keyRequestSize);

With other DRM libraries, usually the key request API requires a PlayReady PSSH box. I've tried this with AMediaDrm_getKeyRequest() but I only get a generic error -10000.

I've also tried the content ID or key ID, as suggested by the documentation, to no avail.

What is the expected init data for the call to AMediaDrm_getKeyRequest() for PlayReady content?