A Unity Timeline Signal extension to create Signals with a String parameter. You can add a Timeline track for this Signal Receiver.

SignalReceiverWithString.cs

using System.Linq;
using System;
using UnityEngine;
using UnityEngine.Events;
using UnityEngine.Playables;
using UnityEngine.Timeline;


public class SignalReceiverWithString : MonoBehaviour, INotificationReceiver
{
    public SignalAssetEventPair[] signalAssetEventPairs;

    [Serializable]
    public class SignalAssetEventPair
    {
        public SignalAsset signalAsset;
        public ParameterizedEvent events;

        [Serializable]
        public class ParameterizedEvent : UnityEvent<string> { }
    }

    public void OnNotify(Playable origin, INotification notification, object context)
    {
        Debug.Log("OnNotify called with notification: " + notification.GetType().Name);
        if (notification is ParameterizedEmitter<string> stringEmitter)
        {
            var matches = signalAssetEventPairs.Where(x => ReferenceEquals(x.signalAsset, stringEmitter.asset));
            foreach (var m in matches)
            {
                m.events.Invoke(stringEmitter.parameter);
            }
        }
    }
}

public class ParameterizedEmitter<T> : SignalEmitter
{
    public T parameter;
}

public class SignalEmitterWithString : ParameterizedEmitter<string>
{
}

[TrackClipType(typeof(SignalReceiverWithString))]
[TrackBindingType(typeof(SignalReceiverWithString))]
public class SignalReceiverWithStringTrack : TrackAsset { }

Add this script to an object that you want to receive signals. Then add a Timeline track called Signal Receiver With String Track. Add the object you added the script to as the binding on the Timeline track. Now you can right-click in the track and add a Signal Emitter with String to the track. This is the Signal that has an added parameter that is a string type. You can easily adapt this to include other types of Signals. Best to separate out this into classes and create other types.

WARNING!! Unity 6 Timeline doesn't seem to like the TrackAsset ... when you set it up, it works, but then, sometime later, and for no apparent reason, the track is disabled and has a warning symbol on it. The signals you added will also be gone. The solution is to just use a normal Signal Track but make sure you also add a Signal Receiver component to the binded object that also has your custom receiver on it.