Don't want to write this by hand? GodCraft sets minecraft:flower_pottable for you with a switch.Get GodCraftOverview
minecraft:flower_pottable is a block component: it goes in the components object of a custom block. Lets the player put this block into an empty flower pot. Combine it with minecraft:embedded_visual to choose how it looks in the pot.
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:flower_pottable": {}
}