Reading the Freshsales documentation, I found really dificult to search for a way to update a deal via PUT on the /api/deals/[id] endpoint. After discovering the contacts_removed_list field, I was able to update a contacts list using the similar field contacts_added_list. So, for appending a contact in a deal one must use the following request(the same holds for creating a deal with a list of of contacts using the correct POST request):
curl -H "Authorization: Token token=[KEY]" -H "Content-Type: application/json" -d '{"deal":{"name":"Test", "amount":100000, "contacts_added_list":[16000148245]}' -X PUT "https://[DOMAIN].myfreshworks.com/crm/sales/api/deals/[id]"
In my opinion this action should be more visible in the Freshsales documentation, preferably in the updating a dealsection, since the information is only visible in this request example:
