Import doesn't works when I use them in the 'startupFcn()' of APP designer

I have imported matlab.http.* and other HTTP packages in the startupFcn, but when other method (function) are called, it still prompts error messages showing that I didn't import them ... I have to import them in EVERY methods that will use those packages... BUT I think this approach sucks ... Anyone who can give me a simpler and reasonable way to solve this? My code is attached below...

startupFcn()

    methods (Access = private)

        % Code that executes after component creation
        function startupFcn(app)
            import matlab.net.*
            import matlab.net.http.*
            import matlab.net.http.io.*

            disp("1. Matlab.net tools loaded ...")

            api_key = string(fileread("KEY.txt"));
            if api_key ~= ""
                disp(["2. API key loaded: "+ api_key]);
            else
                disp("Please Provide your API KEY by editing 'KEY.txt'.");
            end
            
            % Setup hyper parameters when startup
            app.attempt_counter = 0;
            app.max_attempt = 2;
            app.max_tokens = 1024;
            app.n_solution = 1;
            app.prompt = "";

            % Setup HTTP request header
            app.header = [
                HeaderField('Content-Type', 'application/json')
                HeaderField('Authorization', ['Bearer ' + api_key])
            ]';

            disp("3. Header generated: ");
            disp(app.header(2).Value);
        end

Other function that will use HTTP (I have to import them again)

        % Value changed function: ModelDropDown
        function ModelDropDownValueChanged(app, event)
            import matlab.net.*
            import matlab.net.http.*
            import matlab.net.http.io.*

            selected_model = app.ModelDropDown.Value;

            if selected_model == "GPT3.5Turbo"
                app.model = 'gpt-3.5-turbo';
                app.endpoint = "https://api.openai.com/v1/chat/completions";
                disp(app.endpoint);
            elseif selected_model == "GPT3.5"
                app.model = 'gpt-3.5-0301';
            else
                app.model = 'davinci-v3';
            end

            disp("Selected Model is: " + app.model);
        end