Prevent URL-encoding of values when adding headers to TRESTClient - rest

Question

I'm working on a Delphi REST client for a public API that requires an HMAC256/Base64 signed string to be added to the headers of the request to authenticate. I've spent hours trying to figure out why it's not working, so I compared the raw request from my Delphi client to that of a working C# library (using Wireshark).
It turns out my request matches perfectly the request generated by the working C# library, except that Delphi's REST client is URL-encoding the values added to the request's header, therefore invalidating the carefully crafted signature.
This is how I'm adding the signature string to the header:
RESTRequest1.Params.AddHeader('SIGNATURE', FSignature);
The signature string may have slashes, plus signs, and/or equal signs that are being URL-encoded when they shouldn't. For example when the value of the signature string is...
FSignature = '8A1BgACL9kB6P/kXuPdm99s05whfkrOUnEziEtU+0OY=';
...then the request should should output raw headers like...
GET /resource HTTP/1.1
User-Agent: Embarcadero URI Client/1.0
Connection: Keep-Alive
<snip>
SIGNATURE: 8A1BgACL9kB6P/kXuPdm99s05whfkrOUnEziEtU+0OY=
<snip>
...but instead Wireshark shows this as the real value being sent...
SIGNATURE: 8A1BgACL9kB6P%2FkXuPdm99s05whfkrOUnEziEtU%2B0OY%3D
Is there a way to prevent the URL-encoding of values when using AddHeader? Or maybe another way to add raw headers to a TRESTClient request?
PS: I already tried both TRESTRequest.Params.AddHeader and TRESTClient.AddParameter with TRESTRequestParameterKind.pkHTTPHEADER as the Kind parameter. Both resulted in URL-encoded values.
PS2: Using Delphi RAD Studio 10.3.

Answer 1

You should include poDoNotEncode in the Options property of the TRESTRequestParameter.
This can be done using:
RESTClient1.AddParameter('SIGNATURE', FSignature, pkHTTPHEADER, [poDoNotEncode]);
or by using:
RESTClient1.Params.AddHeader('SIGNATURE', FSignature).Options := [poDoNotEncode];