How to create the possibility of sending out unique promotional codes

Hi,

Depends on the process for generating, and later verifying, the codes.

If they need to be taken from a pre-generated list, then you'll need a webhook to "pop" the next unused one off a stack stored in a file.

If it's a more casual situation where you're not worried about the code being generated securely/verifiably, you can generate an alphanumeric code in JavaScript (in the browser) and add that to the form as a hidden field. Only allow that field, call it PromoCodeEvent1982, to be set one time by blocking updates in Admin » Field Management. Let's say a sample PromoCodeEvent1982 value might be "BEBZ".  

Then you can use that to assemble a final unique code. Concatenate

{{lead.Marketo Unique Code}} + 

a static value that represents this particular event, in this case "E1982" + 

{lead.PromoCodeEvent1982}}

That'll give a final code like

XYZPDQ-E1982-BEBZ

It's obviously forgeable because they could hack the "BEBZ" part, but they wouldn't know the rest in advance. 

Again, though, depends on what you actually need to do w/these codes. More typically I see codes with real monetary value, and they need to be assigned from a list of valid codes.