If you're thinking about the Java edition, did you try making a "splashes.txt" file inside a "texts" folder where the Minecraft folder is in your resource pack? Like, here's an example of how it could be formatted:

I am a file and you put documents in me!
Topical!

For Bedrock edition, I'm not too sure if it's even possible.

Here, I created small test resourcepack, you can double-click it to import to your Bedrock Edition - it contains 3 testing custom splashes.

mediafire.com/file/Custom_splashes_test.mcpack