vs2ps VS(
    float4 PosO  : POSITION,
    float4 TexCd : TEXCOORD0)
{
...
}

PosO.x += sin(PosO.y);

PosO.x += sin(PosO.y * Frequency + Phase) * Amplitude;

float Frequency = 10;
float Phase = 0;
float Amplitude = 0.01;
 
vs2ps VS(
    float4 PosO  : POSITION,
    float4 TexCd : TEXCOORD0)
{
    //declare output struct
    vs2ps Out;
 
    //offset x coordinate
    PosO.x += sin(PosO.y * Frequency + Phase) * Amplitude;
 
    //transform position
    Out.Pos = mul(PosO, tWVP);
 
    //transform texturecoordinates
    Out.TexCd = mul(TexCd, tTex);
 
    return Out;
}