I've added the statement to bind the TZ timezone per your suggestion as shown:

WiFiManagerParameter tzParam("tz","Timezone", "UTC0", 50, tz_html);

The Captive Wifi shows 2 TimeZone selection bars

when I press save, the time zone returned is UTC0 (Which I set as the default in the bind) not the US ...

I wrote a captive portal script to capture WiFi credentials, a custom NTP server name, and custom Timezone parameter from a dropdown list to be saved to the NVS. Everything works as expected except the selected timezone parameter is returned as a blank and is not being saved. I am using an ESP32 ...