Apologies for the title which sounds like the Shakira classic, but would you say "Thank you for providing help whenever possible" or "wherever possible"
Both are valid, but they have slightly different connotations.
Help whenever possible is, literally, requesting assistance as often as possible.
Help wherever possible is more equivalent to Help however possible, which isn't asking for a specific amount or frequency of timing on the help, merely asking for help in whatever capacity the recipient can.
Functionally, they both effectively mean, "help as much as possible," and are generally going to be interchangeable, but it is possible that the former could be read as an ongoing request, and the second as a one-time request.