Please use BeatSaber compatible shaders that work with Baked Lighting. You can use the Custom.shader and GlassRefraction.shader shaders from the Beat Saber Modding Group's BS_Shader_Pack (Discord Link).
When you are finished the top Platform GameObject will have a CustomPlatform script already attached to it.
For all the prefabs of objects in your scene to be baked, you must have Lightmap UVs.
To let Unity automatically generate them for you click on each Prefab and in the Inspector under the Model tab, click Generate Lightmap UVs and click Apply. You can click on multiple prefabs to apply this all together.
These following steps are for saving the baked lighting maps into a Prefab in the Custom Platform to be used inside Beat Saber. Lightmapping data is normally exported per Scene (like in VRChat). But a Custom Platform can only export Assets in an AssetBundle so we have to attach all of the data to a Prefab.
In the GameObject with the Custom Platform script, add a new Prefab Lightmap Data component.
Then go to the menu and choose Assets -> Bake Prefab Lightmaps. Once this is done the entries in the Prefab Lightmap Data component will be filled.
The Beat Saber menu light will reflect off of baked lighting while in the menu. Go to the Tweak55 MOD, the UI tab and change Menu Light Color. You can set the Beat Saber menu light to black to turn it off or set it to a certain color.
The colored lights during gameplay will also reflect off the baked lighting. During song selection in the Vanilla Tab in the left menu, choose Colors, Override Default Colors and click the Edit Button beside the row of colors. You can set them to black to turn them off or set the colors to enhance the platform lighting.
Other MODs Can Add Lights Which Affect the Platform
Other MODs can add extra lights into the scene. If they don't have a Culling Mask set to limit Layer 1 they can interfere with baked lighting.
Pictured here is VMCAvatar. If you are using this MOD you can use my VMCLightDisabler to turn off the light whenever the scene changes.