Add a TrustSignature packet to the signature. The values for depth and trust are largely installation dependent but there are some guidelines in RFC 4880 - 5.2.3.13.

public void SetTrust(
	bool isCritical,
	int depth,
	int trustAmount
)

Public Sub SetTrust ( _
	isCritical As Boolean, _
	depth As Integer, _
	trustAmount As Integer _
)

public:
void SetTrust(
	bool isCritical, 
	int depth, 
	int trustAmount
)