KtSkript provides you with a simple scripting solution. It is just a very simple script to display the bossbar. That might be even doable with vanilla commands.
val bossbar = ServerBossBar.builder()
.percent(1f)
.name("&dTest text".t)
.color(BossBarColors.BLUE)
.overlay(BossBarOverlays.NOTCHED_6)
.build()
onPlayerJoin {
bossbar.addPlayer(player)
}