test.cs

[HttpPost]
public IActionResult HandleSignature([FromBody] SignatureData data, string securityToken)
{
    ReturnObject returnObject = new ReturnObject();

    if (securityToken != "123")
    {
        return Ok(returnObject);
    }

    // here, you have access to the signed document and other meta data
    var test = data.SignedDocument.Document; 

    returnObject.Success = true;
    returnObject.Id = data.SignedDocument.UniqueId;

    return Ok(returnObject);
}

public class ReturnObject
{
    public string Id { get; set; }
    public bool Success { get; set; } = false;
}