Attempting to use Crypto APIs to verify JWT signature

I'd like to be able to natively verify a JWT signature using the Crypto APIs, and after reading the API doc for Qc3VerifySignature, I'm not sure how to implement it. According to the doc, parm 1 is the signature, parm 3, I believe is the Base64URL encoded header and payload of the JWT. I'm at the point of receiving CPF9DDB - The key string or Diffie-Hellman parameter string is not valid.

Could someone familiar verify my understanding of the API doc please? Since the tokens are constructed in ASCII, I'm half thinking it's an encoding issue, but before I make too many assumptions, I wanted get other opinions.

Parms:
Signature - Base64Url decoded bytes of the JWT Signature. (Should be Ascii?)
Length of Signature - Length of the Signature string
Input Data - Base64URL encoded string of the JWT header and Payload (Ascii?/Ebcdic?)
Length of Input data - string length of Input data
Input Data Format - 'DATA0100'
Algorithm Description - DataStructure (ALGD0400)
PK Cipher Alg - 50 (RSA)
PKA Block Fmt - '1' (PKCS #1 Block type 1)
Reserved - x'000000'
Signing Hash - 3 (SHA-256)
Algorithm desc Format - 'ALGD0400'
Key desc - Data Structure (KEYD0200)
Key Type - 50 (RSA public)
Key String Len - Length of KeyString
Key Format - '1' (BER string) - Ascii?/Ebcdic?
Reserved - x'000000'
KeyString - BER encoded x.509 certificate (I think this is the x5c value from the OpenID Connect JWKS info)
Key desc Format - 'KEYD0200'
Crypto Provider '0'
Crypto Device Name - *null
Error Code

Thanks,
Doug