Once you discover another Bluetooth device, you can pair with it using a simple API call.
bt_remote_device_t *remote_device;
bool paired = false;
// Search for remote devices.
...
if (bt_rdev_is_paired(remote_device, &paired) == 0) {
if (!paired) {
bt_rdev_pair(remote_device);
}
}
else {
// Handle error
...
}