Image Effect Shader Vertex Data Format

I am trying to use an ImageEffectShader which is applied to the Camera Render Texture via Blit to draw an object into the scene. I am using SRP so I have to rely on the method in this video to apply the Shader to the Camera Texture.

In my vertex shader I am now trying to get the clip space position of a given vertex. To do this, I understand that you have to multiply the vertex data by a projection matrix, but I can't figure out what format the data is originally in and what I am supposed to multiply it with. Especially since this is an ImageEffectShader that gets applied to a texture on a Camera.

When I do this:

 v2f vert (appdata v)
 {
     v2f o;
     o.pos = UnityObjectToClipPos(v.pos);
     o.uv = v.uv;
     return o;
 }

since this is applied to a texture as opposed to an actual object, the v.pos data is not what I expect it to be.

How can I get the actual clip space position of an object based on the data unity provides to an image effect shader?