This is currently not possible in Field Maps. In the future (hopefully later this year) Field Maps will support calculations using Arcade. At this time Arcade does not support making http requests to third party URLs/APIs. This is on the roadmap for Arcade though, so it likely we be possible in the future (possibly this year).
Thanks @Anonymous User !
It would be really handy when that functionality comes in Arcade and Field Maps. That may address some challenges with Survey123 (published with Connect) that any changes in url/syntax need to be made within xls form, not on AGO environment or feature layers/maps directly.
Any update for http requests to third party urls/apis?