This is how i convert the XDR before the initial signature.

    	trustTx, err := build.Transaction(
		build.SourceAccount{recipient.Address()},
		build.AutoSequence{SequenceProvider: horizon.DefaultTestNetClient},
		build.TestNetwork,
		build.Trust(Asset.Code, Asset.Issuer, build.Limit(cd.Limit)),
	)


	var result build.TransactionEnvelopeBuilder
	err := result.Mutate(trustTx)
	if err != nil {
		log.Fatal(err)
	}

	trustTxeB64, err := result.Base64()