Hello @AnthonyA.L ,
At this time coded forms like the Inline and Pop-up forms can't have their thank you messages edited. If you'd like to have an editable thank you page, I'd suggest using the Landing Page sign up forms.
If you're also wanting to completely avoid any redirects, then I suggest adding your coupon code as part of an initial welcome / automation email to the list(s) associated with your inline form.
I've submitted your account to the engineers so if and when they have any updates regarding Inline/Pop-up forms' thank you pages, you'll be notified.
What in your OP did I ignore? I specifically addressed what your workaround options were since the inline and pop-up forms don't have editable Thank You pages at this time, and made sure your account was set up for any future development notification related to your feedback.