I used to be searching for the identical factor as OP and did not discover a solution right here, so this is my answer:

Imports:

utilizing System;
utilizing UnityEditor;
utilizing UnityEngine;

Simplest implementation:

string TextField(string textual content, string placeholder) {
    var newText = EditorGUILayout.TextField(textual content);
    if (String.IsNullOrEmpty(textual content)) {
        var guiColor = GUI.shade;
        GUI.shade = Color.gray;
        EditorGUI.LabelField(GUILayoutUtility.GetLastRect(), placeholder);
        GUI.shade = guiColor;
    }
    return newText;
}

Implementation supporting each TextField and TextArea utilizing generic methodology, in addition to inserting Label on the similar spot the place Text is rendered:

string TextField(string textual content, string placeholder) {
    return TextEnter(textual content, placeholder);
}

string TextArea(string textual content, string placeholder) {
    return TextEnter(textual content, placeholder, space: true);
}

personal string TextEnter(string textual content, string placeholder, bool space = false) {
    var newText = space ? EditorGUILayout.TextArea(textual content) : EditorGUILayout.TextField(textual content);
    if (String.IsNullOrEmpty(textual content.Trim())) {
        const int textMargin = 2;
        var guiColor = GUI.shade;
        GUI.shade = Color.gray;
        var textual contentRect = GUILayoutUtility.GetLastRect();
        var place = new Rect(textual contentRect.x + textMargin, textual contentRect.y, textual contentRect.width, textual contentRect.top);
        EditorGUI.LabelField(place, placeholder);
        GUI.shade = guiColor;
    }
    return newText;
}

Usage:

string textual content = String.Empty;
string space = String.Empty;

void OnGUI() {
    textual content = TextField(textual content, "Field placeholder");
    space = TextArea(space, "Area placeholder");
}

Here’s the way it seems in Unity: