I'd like to trigger a page refresh after a user successfully subscribes

Either by detecting the event or by simply attaching to the final dialog box “Thank you! Your subscription has been created.” with insertAdjacentHTML, I’d like to attach a simple:

<meta http-equiv="refresh" content="1">

Is it possible to do this with a simple script added under Admin > Customize > Themes > Edit CSS/HTML > Head?

Normally it’s possible to add HTML directly to strings under Admin > Customize > Text, which would be the easiest way to solve this, but HTML is disabled on dialog strings, I’m assuming for XSS protection?

What problem are you solving with the page refresh? You might get a better answer if you share your problem rather than your solution.