Custom Widgets cannot upload or use CA signed certificates, and they also cannot call Connector Functions directly. Widgets run inside a sandboxed iframe and only support HTML, CSS, JavaScript, Props, Events, and optional external libraries. TLS settings and certificate handling need to be done in a Connector instead. The supported pattern is to let the widget fire an Event, and then have an App Trigger call the Connector Function. This community thread explains the limitation around calling connectors from inside a widget:
https://community.tulip.co/t/calling-a-connector-function-from-within-a-custom-widget/8551