Use an Item Renamer

/warp how2rename
Go to https://www.birdflop.com/resources/rgb/ or https://minecraft.menu/minecraft-rgb-generator
Use the menu there to create your custom item
Copy the code that it gives you (the stuff in the red rectangle in the above screen shot)
Go back to Mineseed
Make sure that you have the item renamer in your inventory and make sure you are holding the item you want to rename
Do /rename code (paste the code there)
/rename &#FFFFFFE&#FFFBF0x&#FEF6E1a&#FEF2D3m&#FEEEC4p&#FEEAB5l&#FDE5A6e &#FDDD88T&#FDD97Ae&#FCD46Bx&#FCD05Ct &#F7B268G&#F5A36Do&#F29473e&#F08579s &#EB6785H&#E9588Ae&#E64990r&#E43A96e
Enjoy!