This is an issue we’re tracking, and it isn’t caused by anything in your setup. In Settings > Models, clicking the Override OpenAI Base URL field (and the OpenAI API Key field) does not focus them, so you can’t type or paste with the mouse. The toggle still works, which matches what you described.
You’re on 3.15.19, which still has this. Please check for updates (Cursor > Check for Updates). If a newer build is offered, install it and try clicking the field again.
Until then, you can still edit the field from the keyboard:
Go to Settings > Models
Turn on Override OpenAI Base URL, if it isn’t already on
Click a nearby control in that pane, then press Tab until the Base URL field is focused
Type or paste the value (Cmd+V on macOS)
Let us know if anything still blocks you after that.