Based on your title I hope to understand your question correctly. You're currently not inserting the value of the active cell in your url. Try the code below. This code takes the value of the active cell and puts it in the url.
function TransactionVerification() {
var sheet = SpreadsheetApp.getActiveSheet();
var url = "https://api.ethplorer.io/getTxInfo/" + sheet.getActiveCell().getValue() + "?apiKey=freekey"
var response=UrlFetchApp.fetch(url);
var json = response.getContentText();
var data = JSON.parse(json)
sheet.getRange(2,3).setValue([data.success]);
sheet.getRange(2,5).setValue([data.from]);
sheet.getRange(2,6).setValue([data.to]);
sheet.getRange(2,4).setValue([data.value]);
sheet.getRange(2,7).setValue([data.gasUsed]);
Logger.log(data);
From the url in your initial code it almost looks like you want to take the value of the cell in row 2 column 2. If that's the case, change the urlvar to:
var url = "https://api.ethplorer.io/getTxInfo/" + sheet.getRange(2, 2).getValue() + ?apiKey=freekey"