Thanks @Shekhar for pointing me in the right direction!
I added a new file under api/{name}/content-types/{type-name}/lifecycles.ts
export default {
async afterCreate(event) {
const { result, params } = event;
await strapi.entityService.create(
"api::another-type.another-type",
{
data : {
Relation: [result["id"]] // This is how I link the sub-entries with the main entry.
}
});
}
}