Help with Syntax for Basic Math as String

Hello!

I've been struggling with a way for my fiance and I to actually use the Google calendar we share. And by that I mean getting her to actually see it. We fall asleep on the couch quite a bit, so I wanted to set the calendar as our Roku screensaver. This way — we actually pay attention to what's on it.

Fast-forward: It worked! —but I now have one thing I need a bit of coding help with.

I have this bit of code in my screensaver:

m.canvas.SetLayer(1,{   url: "https://api.apiflash.com/v1/urltoimage?access_key=<MYKEYGOESHERE>&format=png&ttl="+rnd(10000).tostr()+"&url=<MYURLGOESHERE>"

I am looking to replace the pseudo-random integer between 1 and 10000 with a pseudo-random integer between 5000 and 10000. I figure I can do this with simple addition by adding 5000 to rnd(5000). Buuuut I can't figure out how to do it while having it embedded in this URL.

In case anyone is curious what the **bleep** I am doing, here's the jist of it:
1. I set up a pretty-looking shared calendar via a (free) service called DAKboard
2. I use a (free) service called API Flash to screenshot my DAKboard URL into a 1080p PNG
3. I use a poorly-coded Roku screensaver to display that image URL

The hangup is that the free version of API Flash has a (totally understandable) monthly limit. I don't need this thing to update more than once every 2-4 hours or so — so I'm cool with using a long Time-to-Live value BUT as we all know if I set a static URL then the Roku cache causes it to not update, well, ever — so I'm using my TTL variable as my cachebuster since adding random flags at the end cause issues with the API Flash syntax.

Anyway - that's it. I like how it ends up looking.

Let me know if you can help me with math coding!