This is how you can sign a message with an embedded RPC server in frame.sh using https://github.com/defiweb/go-eth

sign-with-frame-sh.go

package main

import (
	"context"
	"fmt"

	"github.com/defiweb/go-eth/rpc"
	"github.com/defiweb/go-eth/rpc/transport"
	"github.com/defiweb/go-eth/types"
	"github.com/defiweb/go-eth/wallet"
)

func main() {
	t, err := transport.NewHTTP(transport.HTTPOptions{URL: "http://127.0.0.1:1248"})
	if err != nil {
		panic(err)
	}
	c, err := rpc.NewClient(rpc.WithTransport(t))
	if err != nil {
		panic(err)
	}
	a := types.MustAddressFromHex("0x1234567890123456789012345678901234567890")
	m, err := wallet.NewKeyRPC(c, a).SignMessage(context.Background(), []byte("data"))
	if err != nil {
		panic(err)
	}
	fmt.Println(m)
	s, err := c.Sign(context.Background(), a, []byte("data"))
	if err != nil {
		panic(err)
	}
	fmt.Println(s)
}

Of course, this requires you to have the key (signer) for the above mentioned address configured in frame.sh