WeTravel does not have a full "discount/promo code" feature yet, it will be introduce in summer 2017.
However, WeTravel has "custom prices" which can serve a similar purpose. After a participant pays the deposit, you can set a "custom price".
Let's say you give a participant a $100 discount on a $1000 trip with a $200 deposit. After they pay the deposit, you simply click on "custom price" next to their name.
Their, set the price to $900.
This will automatically adjust their balance due and send them an email, inform them about the new price of $900. The email also includes a link where they can pay the balance due.
If you need to find out which participant to give a discount to, you could add a question in the "Traveler Info" field, adding adding a question saying:
"Discount Code (applied after deposit it paid)"
This way, you will see in your traveler information whom to give the "Custom Price" to.
Let me us know if you would like further info.