Using the REST API to update leads, got "field not found" error

We have custom fields for leads like UTM Source and UTM Medium.

I want to use the REST API to update these fields for leads.

I tried using the /rest/v1/leads.json endpoint with this request:

POST /rest/v1/leads.json HTTP/1.1
Content-Type: application/json
Authorization: Bearer TOKEN
{"action":"updateOnly","lookupField":"id","input":[{"id":LEAD_ID,"UTM Medium":"MEDIUM","UTM Source":"SOURCE"}]}

But I got this response saying the fields were not found:

{"requestId"=>"143e9#1497880b900",
 "result"=>
  [{"status"=>"skipped",
    "reasons"=>
     [{"code"=>"1006", "message"=>"Field 'UTM Medium' not found"},
      {"code"=>"1006", "message"=>"Field 'UTM Source' not found"}]}],
 "success"=>true}