Don't want to write this by hand? GodCraft sets minecraft:input_ground_controlled for you with a switch.Get GodCraftOverview
minecraft:input_ground_controlled is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets a rider steer the entity on the ground with the movement keys. Rideable entities can only step up half a block by default, so add minecraft:variable_max_auto_step to climb higher.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:input_ground_controlled": {}
}