@HugeG you can use the below codes. Please note that 1. The code you were using was not correct, phprad does not generate and id called id. And 2. PHPRad does not use input with type submit in its forms, rather PHPRad uses a button that has type submit. And also when you get codes from the internet, try to understand them so that you know how to properly use them in your phprad projects.
Use the below code to target a specific form.
$('form#{table-name}-add-form').submit(function(){
$(this).find(':button[type=submit]').prop('disabled', true);
});
In the above code replace {table-name} with the name of the table where the page you are working on exist.
And use the below code to apply it to all forms(i.e to target all form submit button).
$('form').submit(function(){
$(this).find(':button[type=submit]').prop('disabled', true);
});