Please see this screenshot I made for you to see how it works.
<https://i.postimg.cc/C1PWHqMm/imhere.jpg>

So that others can benefit from the information, there's an iOS/Android app
that can geofence multiple areas and send texts when you're leaving or
entering custom areas, which you simply draw onto an OpenStreet map.

https://apps.apple.com/us/app/im-here-zone/id1452495641?ls=1
https://play.google.com/store/apps/details?id=com.syncitgroup.android.iamhere

https://www.iamherezone.com/ by SyncIt
"This app allows users to send pre-generated messages based on
their geofence location area, thus informing other users when
they enter or leave a geographical area. The user defines the
geographical area by drawing it on the map. The app supports
SMS messaging, WhatsApp, email, and push notifications."

1. Draw a zone (on OpenStreetMap)
2. Write your text message
3. Select recipients

Create multiple zones
Store multiple recipients
Send default and customized messages
Setup recurring and one-time messages
Add icon and color to each zone

Duration selectable for 1h, 2h, 4h, 8h, 12h, 1 day & recurring.

The app only has a thousand downloads, so you'd have to be good at
searching to find it (luckily, I have the best app search engine tools).

The caveat is that I only briefly tested it with mock location settings on
Android so that I could make those screenshots for you to take a look at.

As a result, I can't vouch for how well it might work in the real world.

However, if others find the functionality useful, let the rest of us know
so that we all benefit from the effort you invested in testing the app out.