How can I create a SMS flow based on a specific subscribe keyword?

I’m trying to set up a SMS subscription keyword that will trigger a different welcome message than my normal one -- essentially having customers text in a keyword that will 1) add them as subscribers, 2) send them a custom message with a coupon code and a link to the product they’re texting about.

I can’t set up a flow that’s triggered by becoming a new subscriber + filter for the keyword in their message, since those are two different metrics (became subscriber + sending SMS), and filtering for the keyword in their message doesn’t seem to work in the tests I’ve done. If I make an automated keyword in Settings, I can’t add a link to the special product. 

So far, it looks like the best I can do is to make a new subscriber/welcome flow that’s only live for a short period of time, which is a huge hassle. Our texting number is verified and has been used previously, so that’s not the issue.  Has anyone else had this issue and figured out a way around it?