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minecraft:spawn_egg_interaction is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets players use this mob's own spawn egg on it to spawn a baby. The baby gets the entity_born event plus any on_spawn event you set.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:spawn_egg_interaction": {}
}