Surprising to me a lot of people do not know how to make a EBOOT start a SPRX, I'll show you how and it's actually REALLY easy to do.
1: Get an EBOOT to a game of your choice, lets say we want to have GTA V start the SPRX menu called "Extortion" get a DEBUGGED EBOOT for GTA V and put it in the same folder as TrueAncestor and the SPRX builder
2: Start TrueAncester, press "4" and then "ENTER" on your keyboard to decrypt it into a .ELF which will cause an EBOOT.ELF to be created in the same folder.
3: Start the SPRX Elf Builder, press the "..." button, browse into the same folder and choose the newly created "EBOOT.ELF" once it is opened, in the box that says "MyPlugin" change that to whatever it is that the SPRX is called, I will make mine say Extortion. Click on the build button and you will see a new file has been created called "sprx_EBOOT.ELF"
4: Delete the EBOOT.BIN and EBOOT.ELF, then rename the "sprx_EBOOT.ELF" to "EBOOT.ELF"
5: Start TrueAncestor again and this time press "5" and then "ENTER" on your keyboard which will rebuild the "EBOOT.ELF" into a "EBOOT.BIN" that your PS3 will read.
THAT'S IT! The EBOOT that has just been created will launch the SPRX that you chose the name of in the SPRX builder, it will launch the SPRX from the dev_hdd0/tmp folder in your PS3 when you start the game once you have put the EBOOT inside the games folder.