I want message to show when there is a specific number of seconds on the timer
You will need to find an online service to send these text messages on your behalf.
If you find a suitable API, you can let the user sign up to receive text messages in the app, then send the relevant information to your API, and let the service send text messages at your scheduled times.
If you search for schedule SMS API online, you should find some relevant APIs.
You can add an API to your project using the web API component.
You can see an example of using the Web API component here: Reverse Geocoding App - Get Location from Latitude/Longitude