Follow first `message://` URL in text field?

I’m sure there is.

As always, you will need to provide sample input, to make testing possible.

( showing works better than telling )

In the meanwhile, you could experiment with the following version of the code in the Execute JS action:

Expand disclosure triangle to view JS source
kmvar.local_Clip.split("message://")
.slice(1)
.map(x => {

    const
        match = x.split(/[\s\\)]/ug)
        .filter(Boolean)[0];

    return `message://${match}`;
})
.join("\n");