We are trying to build a jmeter testcase which does the following:
I was able to solve it myself. The solution is as follows:
In it, the following code displays the captcha and waits for user input
filenameOrURL = new URL("${captchaimage}");
image = Toolkit.getDefaultToolkit().getImage(filenameOrURL);
Icon icon = new javax.swing.ImageIcon(image);
JOptionPane pane = new JOptionPane("Enter Captcha", 0, 0, null);
String captcha = pane.showInputDialog(null, "Captcha", "Captcha", 0, icon, null, null);
Then we can use the captcha variable in any way we want. Thank you everyone who tried to help.