Google Sheets – Find and Replace Text

You don't need a script for this. Enter the following in C1:

=arrayformula(iferror(regexextract(B1:B, "ID#\d+")))

This will populate column C with Id numbers (where the data in B is available), leaving the rest blank. As more entries are submitted through the form, more Id numbers will be in column C.

Another version uses a second regexextract to drop the ID# part and keep only the digits:

 =arrayformula(iferror(regexextract(regexextract(B1:B, "ID#\d+"), "\d+")))

If you highlight the column & press Ctrl-H you should be able to replace the string MKE how you want.

Before:

After:

Edit:
To auto fire this replacement:

In your sheet choose Tools|Script Editor from the menu & paste the code below which act on cells c1 to c10 on the sheet. The OnOpen method should fire when the sheet is opened, or can be fired directly from the script editor. I've not changed the UI to add the function to the menu, but that is also possible.

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getRange("c1:c10");
  var to_replace = "MKE";
  var replace_with = "Milwaukee, WI";
  replaceInSheet(sheet,range, to_replace, replace_with);
}

function replaceInSheet(sheet, range, to_replace, replace_with) {
  //Confirm
  var ui = SpreadsheetApp.getUi(); 
  var spread = SpreadsheetApp.getActiveSpreadsheet();

  var result = ui.alert(
     "Will update " + to_replace + " to " + replace_with + " ",
     'Are you sure you want to continue?',
      ui.ButtonSet.YES_NO);

  // Process the user's response.
  if (result == ui.Button.YES) {

    // User clicked "Yes".
    spread.toast("Will update " + to_replace + " to " + replace_with + " ", "ALERT");

    var data  = range.getValues();

    var oldValue="";
    var newValue="";
    var cellsChanged = 0;

    for (var row=0; row<data.length; row++) {
      for (var item=0; item<data[row].length; item++) {
        oldValue = data[row][item];
        newValue = data[row][item].replace(to_replace, replace_with);
        if (oldValue!=newValue)
        {
          cellsChanged++;
          data[row][item] = newValue;
        }
      }
    }
    range.setValues(data);
    spread.toast(cellsChanged + " cells changed", "STATUS");
  }
  else {
    // User clicked "No" or X in the title bar.
    spread.toast("No action taken", "ABANDONED");
  }
}