How to use the taskcluster.utils.encodeStringForB64Header function in taskcluster

'clientId:' + utils.toStr(name),
            'issuer:' + utils.toStr(clientId),
        ])
    sig.extend([
        'seed:' + utils.toStr(cert['seed']),
        'start:' + utils.toStr(cert['start']),
        'expiry:' + utils.toStr(cert['expiry']),
        'scopes:'
    ] + scopes)
    sigStr = '\n'.join(sig).encode()

    if isinstance(accessToken, six.text_type):
        accessToken = accessToken.encode()
    sig = hmac.new(accessToken, sigStr, hashlib.sha256).digest()

    cert['signature'] = utils.encodeStringForB64Header(sig)

    newToken = hmac.new(accessToken, cert['seed'], hashlib.sha256).digest()
    newToken = utils.makeB64UrlSafe(utils.encodeStringForB64Header(newToken)).replace(b'=', b'')

    return {
        'clientId': name or clientId,
        'accessToken': newToken,
        'certificate': utils.dumpJson(cert),
    }