How do you enchant a stick in Minecraft Bedrock Edition version 1.9?

Unfortunately, this is not possible.

From the wiki:

Although /enchant enforces maximum levels and compatibility, other commands (such as /give, /replaceitem, and /data) can bypass these restrictions.

However, the give and replaceitem commands only support the JSON tags can_place_on and can_destroy. See this for more information on that. The data command isn't available in Bedrock Edition/Pocket Edition.

You can mod any enchantment using the Minecraft Universal Editor. The easiest way is to put a enchanted sword and the not enchanted stick in a chest. Go into the Editor, search the chest and copy the enchantment over to your stick. I did it right now.

I think you can only do a can_place_on or can_destroy:

/give @p iron_pickaxe 1 0 {¨can_destroy¨:{¨blocks¨:[¨minecraft:coal_ore¨]}}

That would give you an iron pickaxe that could break coal in adventure mode. You could also do:

/give @p deadbush 1 0 {¨can_place_on¨:{¨blocks¨:[¨minecraft:grass¨]}}

That would give you a deadbush that you can place on grass in adventure mode.