Achievement creator?
Can you create achievements for your own puzzles?
Short story: no.
In puzzle creator: no
In hammer: not really, though you could use some custom signage at the end of a level to show if people achieved the achievement(s) in your map.
To actually create achievements, you would have to create a mod. A real, game code editing mod.
And because the code for portal 2 isn't published it is impossible at the moment. 
or, you could create vgui that is parented to the player's veiwpoint that displys after the trigger
Aperture Science: We do our science asbestos we can!
In order to create a vgui you need to add game code. Just putting it out there...
More than just a mod, you had to have a mod that was given accss, unless they give out Steamworks access to any schmuck these days. Back when I remember doing this stuff you had to have access to the Steam achievement server.
Please do not Private Message me for assistance. Post a thread if you have questions or concerns.
If you need to contact the staff privately, contact the Global Moderators via Discord.
mmmmm. Isn't there games where there is achievements inside it but not on the steam page? Guessing this is what the OP wanted.