Using the DropBox API to host files for a public website (.net 9 Blazor)

My client has 100,000s of images stored in dropbox.  The goal is to have some database information displayed alongside any one of the images.

From what I understand, I would need need to use a service account or OAuth2 so that the visiting user never knows anything about the file location and the only access is via the owners dropbox authority via the app.

I have tried to follow the online instructions but they are varied and change from site to site and over the years.

My first step was to load this url - https://www.dropbox.com/oauth2/authorize?client_id=&token_access_type=offline&response_type=code

This returned a Code.  I placed this code the below .net 9 Blazor app but I cannot get past Bad Request

refreshToken = the code I got from the above?

public async Task<string> RefreshAccessTokenAsync(string refreshToken)
{
    var client = _httpClientFactory.CreateClient();
    var request = new HttpRequestMessage(HttpMethod.Post, "https://api.dropbox.com/oauth2/token")
    {
        Content = new FormUrlEncodedContent(new Dictionary<string, string>
            {
                { "grant_type", "refresh_token" },
                { "refresh_token", refreshToken },
                { "client_id", "xxxx" },
                { "client_secret", "xxxx" }
            })
    };

    var response = await client.SendAsync(request);
    response.EnsureSuccessStatusCode();

    var json = await response.Content.ReadAsStringAsync();
    var tokenResponse = JsonSerializer.Deserialize<JsonElement>(json);
    return tokenResponse.GetProperty("access_token").GetString();
}

Are there any step by step examples that work to do the above in 2025?  I'm really banging my head against the wall here.