Make Local variable share between only 2 functions

Not sure i understand ur question exactly...

A function or method are a way to control a variable if need be.

If you want a value to never change you can make it constant like

 const string KEY_MYKEY = "MEYKEY";
 const int CONST_VALUE_1 = 10;
 const float CONST_VALUE_2 = 8.65f;

or simply set a value and Not change it in any function or method and make something else = it's value

 public string newString = "Don't Change";
 public void OnGetStringValue(string messageToChange)
 {
     messageToChange = newString;
 }

alternatively you can use a get method also

 string newString = "";
 void Awake()
 {
     SetDontChangeString = "Don't Change";
 }
 public string GetDontChangeString { get { return newString; } }
 public string SetDontChangeString { set { newString = value; } }
 public void OnChangeMessage(string messageToChange)
 {
    messageToChange = GetDontChangeString ;
 }

what exactly is your goal?

We can do other things like make lists of word to display in the scene.

 //Requires 'using UnityEngine.UI;' at the top outside class to use
 List<string> listOfWords = new List<string>()
 {
     "Start",
     "Login",
     "Logout",
     "Setting",
     "Options",
     "Quit",
 }
 GameObject prefabButton;
 
 //should populate the panel with a bunch of prefabs with the string changed. Usually i use a ScrollView and enter in it's content GameObject. I'll then add a vertical or horizontal layout group to it and adjust settings according to my prefabs.
 public void DisplayOptions(GameObject contentPanel)
 {
     for(int i = 0; i < listOfWords.Count; i++)
     {
         GameObject obj = Instantiate(prefabButton, contentPanel.transform);
         obj.name = listOfWords[i];
         //Change Name of Text in find if you have changed the button text Component name if you used a default unity button.
         obj.transform.Find("Text").GetComponent<Text>().text = listOfWords[i];
     }
 }