Unlike Minecraft Java Edition, Bedrock servers have no native /worldborder command. To enforce a boundary, you have to restrict players' movement in a different way.
Options
Command block
With a command block in a loaded chunk you can teleport players back to the center whenever they cross a certain coordinate. Example – pull all players further than 500 blocks from 0, 0 back to the center:
/execute as @a[rm=500] run tp @s 0 ~ 0
The command block must run in Repeat mode with Always Active enabled so the command is executed continuously.
Behavior pack
You can create or install a behavior pack that enforces the desired boundary – for example with damage outside the area or an automatic teleport. Upload behavior packs as described in Add Behavior and Resource Packs and enable them for the world.