Can't get a "Hello World" custom shader to work

Phaser 3
1

Title. I made a super basic custom shader:

import 'phaser';

export class TestShader extends Phaser.Renderer.WebGL.Pipelines.PostFXPipeline {
    constructor(game: Phaser.Game)
    {
        super({
            game, 
            fragShader: `
                #DEFINE SHADER_NAME TEST_SHADER

                #ifdef GL_ES
                precision medimp float;
                #endif

                varying vec2 outTextCoord;
                uniform sampler2D uMainSampler;

                void main() {
                    gl_FragColor = vec4(1.0, 0.4, 0.723, 1.0);
                }`
        });
    }
}

and use it in a super basic way:

this.cameras.main.setPostPipeline(new TestShader(this.game));

and yet I still see no difference in my scene. Any idea what is going wrong? I literally can’t simplify this any further to test it.