How do I send participants into a Qualtrics survey after they finish their IAT tasks?

See https://www.millisecond.com/support/docs/v5/html/howto/interopsurveys.htm under "Returning the Subject ID from Inquisit Back to 3rd Party Site" or "Sending the Subject ID from Inquisit to 3rd Party Sites" depending on your setup.

What if I have multiple surveys for multiple conditions (i.e. I need to assign one survey to one group and a different one to another)? Based on those instructions, it seems like I can only set one "Finish page." 

I can also accomplish what I need to by directing only certain groups to the survey site through the Finish page, and direct others to the default Finish page. Is there a way to do this?

Then you need to set the finish URL in the script by setting the defaults.finishpage property depending on the group. See e.g. https://www.millisecond.com/forums/FindPost22063.aspx

The URL for the default finish page is https://www.millisecond.com/web/finish5.aspx

Is there a way to set the finish URL in the script without making it dependent on the group?

You can set it based on any condition or no condition at all.

Would adding the following line to my experiment accomplish this, then?

/ onexptend = [defaults.finishpage = "https://yourwebsitehere.com?subjectid=<%script.subjectid%>"]

Almost. You need to put the URL into a <text> element, though. I.e.

<text finishURL>
/ items = ("https://yourwebsitehere.com?subjectid=<%script.subjectid%>")
</text>

with

/onexptend = [defaults.finishpage =defaults.finishpage = text.finishURL.item(1);]

Thanks so much for your help! I've implemented this into my code. 

When I run it on Lab, though, it doesn't seem to be taking me to my URL. Is this simply because I'm working from a laptop with an expired Lab trial rather than my desktop in the office? Or should I be looking for some sort of error I've made?

Inquisit Lab doesn't do any forwarding, finish pages only apply when running under Inquisit Web.