How to add new entry that have a relation fields - of many to one with api

i try to insert new entry to my collection : article
and i have a relation with other collection - Sites (article can be associated to many sites)
i also have a swagger documentation that explain the request body,
by swagger i need to send array of id’s or string


this is my request body :
{
“data”: {
“Title”: “10 Years Younger”,
“name”: “tips-tricks-look-younger-syn”,
“primary_site”: “localhost:3000”,
“slug”: “trending”,
“sites”: [12] → id of site on my local strapi
}
}
and i got this error :
{
“data”: null,
“error”: {
“status”: 400,
“name”: “ValidationError”,
“message”: “1 relation(s) of type api::site.site associated with this entity do not exist”,
“details”: {
“errors”: [
{
“path”: ,
“message”: “1 relation(s) of type api::site.site associated with this entity do not exist”,
“name”: “ValidationError”
}
]
}
}
}