Retrieve content from a text file and assign it to a TextMesh component.

SetTextFromFile.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using TMPro;

[RequireComponent(typeof(TextMesh))]
[ExecuteInEditMode]
public class SetTextFromFile : MonoBehaviour
{
    public TextAsset textFile;

    [SerializeField]
    private TMP_Text textMesh;

    void Update()
    {
        if (textFile != null)
        {
            textMesh.text = textFile.text;
        }
    }

    void OnValidate()
    {
        textMesh = GetComponent<TMP_Text>();
        if (textFile != null)
        {
            textMesh.text = textFile.text;
        }
    }
}