Custom HTML Form Auto Close Popup


I am developing an attended automation which uses a custom HTML input form to get transaction data from the user. This input dialog should time out, close and throw a BRE after one minute if the user did not input a value so I have added a self.close property within my HTML to achieve this.
When the form has been opened for a minute and attempts to close, I encounter a popup window asking “Do you want to close this window?”. I haven’t been able to deal with this pop up in my robot process as no activity will run until the form is completely closed. I have to close this manually to continue running my process.
Any ideas on how I could deal with this pop up? Would there be a way to prevent it from appearing at all?