How to get the screen position of a point drawn in the skybox?

[tankabeko]

Hello.

I want to get the screen position of a point on the skybox and display text there.
For example, here is the code

Shader "CustomSkybox/Sun"
{
    Properties {
        _BGColor ("Background Color", Color) = (0.7, 0.7, 1, 1)
        _SunColor ("Color", Color) = (1, 0.8, 0.5, 1)
        _SunDir ("Sun Direction", Vector) = (0, 0.5, 1, 0)
        _SunStrength("Sun Strengh", Range(0, 300)) = 12
    }
    SubShader
    {
        Tags
        {
            "RenderType"="Background"
            "Queue"="Background"
            "PreviewType"="SkyBox"
        }

        Pass
        {
            ZWrite Off
            Cull Off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            fixed3 _BGColor;
            fixed3 _SunColor;
            float3 _SunDir;
            float _SunStrength;

            struct appdata
            {
                float4 vertex : POSITION;
            };

            struct v2f
            {
                float4 vertex : SV_POSITION;
                float3 viewDir : TEXCOORD0;
            };

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.viewDir = -WorldSpaceViewDir(v.vertex);
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                float3 sunDir = normalize(_SunDir);
                float3 dir = normalize(i.viewDir);
                float angle = dot(dir, sunDir);
                fixed3 c = _BGColor + _SunColor * pow(max(0.0, angle), _SunStrength);
                return fixed4(c, 1.0);
            }
            ENDCG
        }
    }
}

This shader specifies the position of the sun in 3D coordinates from the inspector, normalizes it on a sphere, and draws the sun by dot product with the world space view vector.

What I want to do is to get the screen position of the pixel whose dot product is zero.
If I can get those coordinates I would be able to draw the text. However, I do not know how to do that.

Ultimately this is what I would like to do. If there is a way to achieve this outside of shaders, please let me know.

Thank you in advance.