Welcome to the community, @Georgina_Oxford!
Unfortunately, one of Airtable’s gigantic limitations is that it can’t trigger automations based on the presence of information in a field, because Airtable isn’t smart enough to wait until you are finished typing in information into a field. Airtable will trigger as soon as you type the very first letter into your URL field.
So you would need to add a checkbox field to each record.
When you are ready to send the email, you would check the checkbox field which would trigger your automation.
You would setup your automation’s trigger to trigger when the checkbox is checked, and then setup your action to send an email.