Invalid URI format with HttpRequestMessage used by HttpClient

In a .NET 6 Web API, in program.cs I have this:

The baseAddress content is : https://localhost:7122/ (coming from config file, baseAddress value is ok)

builder.Services.AddHttpClient<IMyService, MyService>(client => client.BaseAddress = new Uri(baseAddress));

When I execute the code below I get an error "'Invalid URI: The format of the URI could not be determined.' when I create the object HttpRequestMessage. I need to send an object when I do _httpClient.SendAsync

In MyService class

public async Task<MyResponse> Search(MyModel myModel)
{
    var httpRequest = new HttpRequestMessage
    {
        Method = HttpMethod.Get,
        RequestUri = new Uri("MyAction/Get"),
        Content =
            new StringContent(
                JsonSerializer.Serialize(myModel),
                Encoding.UTF8,
                MediaTypeNames.Application.Json),
    };

    var response = await _httpClient.SendAsync(httpRequest).ConfigureAwait(false);
    response.EnsureSuccessStatusCode();

    var responseBody = await response.Content
        .ReadFromJsonAsync<MyResponse>()
        .ConfigureAwait(false);

    return responseBody;
}

When I replace RequestUri by this : RequestUri = new Uri($"https://localhost:7122/MyAction/Get") that's work.

Am I missed something ?