Create a webapi project, e.g.:

dotnet new webapi -o MyMicroservice

Add a middleware class:

public class ApiKeyMiddleware
{
    private readonly string _apiKeyName;
    private readonly string _apiKeyValue;
 
    private readonly RequestDelegate _next;
 
    public ApiKeyMiddleware(RequestDelegate next, string apiKeyName, string apiKeyValue)
    {
        _next = next;
        _apiKeyName = apiKeyName;
        _apiKeyValue = apiKeyValue;
    }
 
    public async Task InvokeAsync(HttpContext context)
    {
        if (!context.Request.Headers.TryGetValue(_apiKeyName, out var extractedApiKey))
        {
            context.Response.StatusCode = 401;
            await context.Response.WriteAsync("API Key was not provided.");
            return;
        }
 
        if (!_apiKeyValue.Equals(extractedApiKey))
        {
            context.Response.StatusCode = 403;
            await context.Response.WriteAsync("Unauthorized client.");
            return;
        }
 
        await _next(context);
    }
}

Handle the API key check in Program.cs with the following code. Place this before the app.MapControllers() call:

string? apiKeyName = "My-Api-Key-Name";
string? apiKeyValue = "85a80751-cc34-4a6e-9ad9-4d9c9bea403c";  // Do NOT store your actual key value inline!
                                                               // Retrieve it from a protected
                                                               // location, e.g., an Azure key vault.
 
// Validate the request:
app.UseMiddleware<ApiKeyMiddleware>(apiKeyName, apiKeyValue);

Calls to the API look like this:

GET https://your_webservice_url/some_endpoint
Accept: application/json
My-Api-Key-Name: 85a80751-cc34-4a6e-9ad9-4d9c9bea403c