Using Javascript check/uncheck all button to work on SiteOrigin contact form widget

Hello,
I’m using the contact form widget to have people select some services they’d like to know more about from a checkbox list of around 5 services. At the bottom of the list, i’d like to have the button “check all” and the button “uncheck all”, to let them check or uncheck all services. Ideally, when the users clicks on the “send” button to send the message, I should receive an email with this selection, just like it works now if you manually select each service.
I’ve added a .js file into the wp-includes/js directory, and I’m trying to call this action from a Plain Text widget that I inserted under the Contact Form Widget. The buttons don’t show up, and the “recall” script I inserted into the Text widget gets changed in a weird way each time I save the page.
I’m not a Java developer and not familiar with this, so I must be doing something very wrong and hopefully someone could help me with a step-bystep-for-dummies instructions to achieve this. Thank you!!!