Because of the way Wappler is made, you can't do this just like you'd do in PHP.
To run JavaScript, best thing here is:
Edit: Regarding the PayPal SDK, you'd have to build a custom extension for that, as RunJS doesn't allow loading external modules. RunJS could theoretically do so, but JonL chose not to support it due to security concerns (risk of people not knowing what they're doing and compromising the security of their app and then blaming Wappler)