Don't want to write this by hand? GodCraft sets minecraft:friction for you with a switch.Get GodCraftOverview
minecraft:friction is a block component: it goes in the components object of a custom block. Sets how slippery the block is to walk on, from 0.0 to 0.9. Higher numbers give more grip and slow movement more, lower numbers make it more slippery.
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:friction": {}
}