How to test a relationship in seeds with a source() in dbt - testing

Question

There is any way to test a seed with a relatioship coming from a external source?
I'm trying to test the seeds before creating them, to do this I'm creating in this moment a relationship test but with an external source.
I would like to do something like this in the seeds/schema.yml:
version: 2
seeds:
- name: products_market
description: ""
columns:
- name: id_source
description: "ID source"
- name: id_target
description: "id target"
tests:
- relationship:
to: source('dt', 'products')
field: id_product
In this case the field id_target would be checked if exists the field id_product from dt.products
But I get the error
'test_relationship' is undefined. This can happen when calling a macro that does not exist. Check for typos and/or install package dependencies with "dbt deps".
The sources.yml
version: 2
sources:
- name: dt
tables:
- name: products
description: General Products
I saw this info but not sure if I'm doing it well or maybe there is another way to do this
https://docs.getdbt.com/reference/seed-properties
https://docs.getdbt.com/reference/resource-properties/tests
EDIT:
I was missing an 's' in the test relationships statement in the schema.yml. Anyway it doesn't look i'm doing it properly, it says there is no test or do nothing when I do dbt seed... or dbt build
version: 2
seeds:
- name: products_market
description: ""
columns:
- name: id_source
description: "ID source"
- name: id_target
description: "id target"
tests:
- relationship**s**:
to: source('dt', 'products')
field: id_product
Thanks in advance