Hi,
I have not tried using SignedXML Codeunit, but I gone through these functions while working on a project.
You can try to use these functions from this CU.
InitializeSignedXml(SigningXmlDocument: XmlDocument) // Your document that need to be signed
SetSigningKey(XmlString: Text) //This is the key used to sign your document like at the end
GetXml(): XmlElement // To get the signed xml back
//
<RSAKeyValue><Modulus>vIVB/S+boTqUH03VpVw8cIJ9XkXXirW/iXIJgdYUnkgpEglWP7w6ZWvpOQ+xdtc7u4Fn5w2zlOgkvjvGtGWjLR/OGjMILPIaTcKalCP9ccuSIF573VkYps5FGrvNpQrrJ4Kl10VW4vvw6db6RWdGWD0sS7UV0Twh66+tSLA+LX2nuHCv0S5ob5rQbxLoL5Py4kOUFoQBidvpk4MMIDv9Y/+RCzkpibyyQnCpv3jOBkRmLbHu9eaoRsc+hD4I2Lau5wg2F4ZhNw2qCVxtW5hLmPvxdNqm+3CP79Ba9L86+ZUsHvChPm6otfTwXrGEQEAagmo9Bn7xghK1d4Ngihc/Nw==</Modulus><Exponent>AQAB</Exponent>
........
6XHMRPXrBmWISUvHimmK0L1r7D/lFBkRPlDFnCa1JxsuC2urYcSDcB7SWmhQFMCz6v7fnqgZzm0ukCDYkRhVhfeGL9bG+64ObsSgG3vhmhjJoduAjQ+LE1jpuPZBQ71KT6dlr4KllJxmSFWcNj+L1HrTqskyB6502JXOKsQJhzQaw8vOAk/+/bwzkJ5+UeoQ==</D></RSAKeyValue>
//