How to make a shader that always turns into the direction of the camera?

Code (CSharp):

1. v2f vert (appdata v)
2.             {
3.                 v2f o;
4.                 o.vertex = UnityObjectToClipPos(v.vertex); //guess here should happen something
5.                 o.uv = TRANSFORM_TEX(v.uv, _MainTex);
6.                 return o;
7.             }

this should help also: https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html

but I have no idea how to make it happen.

 

Search for billboard shader.