Create a widget/shortcut to an app URL? Whatsapp allows to send a message to any number if I browse to https://wa.me/. That displays a web page that has a button that navigates to 'whatsapp://send?phone='.
I want to have a shortcut that asks for the number and then opens the whatsapp:// link (without going through the web page).
I tried to use 'HTTP Request Shortcut' (https://play.google.com/store/apps/details?id=ch.rmy.android.http_shortcuts), but it only accepts http links and then just shows the response. Tried to use some activity shortcuts, but I can't find one that has the whatsapp activity to open a chat with a prompt to a number.
I'm the creator of the HTTP Request Shortcuts app, and I believe it can be used for what you're describing. You can try this:

Open the HTTP Shortcuts app
Click the + button at the bottom right to create a new shortcut
In the dialog that pops up select &quot;Scripting Shortcut&quot;
Give the shortcut a name (and a description and different icon if you want)
Click on &quot;Scripting&quot; to open the Scripting editor
Enter the following JavaScript code:
const number = prompt('Enter the number');
sendIntent({
type: 'activity',
action: 'android.intent.action.VIEW',
dataUri: 'whatsapp://send?phone='+number,
});
Save your changes.

You can then run this shortcut from within the app, or place it on your home screen. When you click it, it should prompt you to enter a number, and will then invoke the URL 'whatsapp://send?phone=' with the number appended.
Hope this helps. For more information regarding Scripting in the HTTP Request Shortcuts app see the official documentation here: https://http-shortcuts.rmy.ch/scripting