The numbers go in the query string, comma-separated, as strings of digits:
curl -s -X DELETE -H "Authorization: Bearer $FIRMUS_KEY" \
"https://api.firmus.sh/v1/fr/watchlists/1/watches?numbers=552032534,55203253400041"numbers is required, although the contract below marks it optional: a comma-separated
string of up to 1 000 numbers, sent as you received them, leading zeros kept. Without it
the answer is 400 invalid_request.
Each number answers removed, not_watched (not in this list) or invalid, in the order
sent, with counts, the list's watched total and the allowance. A number that no other
list of the organization watches frees its unit at once. Not counted as a call.