Hi,
Step 1: Set this code on OnVisible property of Catpcha Screen:
Set(captcha_var,Left(GUID(),8))
Step 2: Use captcha_var in Text property of label

Step 3: Use input text to validate
If(Label18.Text=TextInput3.Text,Notify("Successfull login !!"), Set(captcha_var,Left(GUID(),8)) )

Manually Refresh Captcha
Use same variable to refresh captcha

If this solves your problem, then please Accept it as Solution.