I think this problem (or very similar problem) has been discussed here many times in the past, but just to be sure - can you post an image or url to something similar?
Have you made an attempt yet - if so can you attach the ipt file here?
Also - what version of Inventor did you say you are using?
Attached is an example file that you can look at. Note there might be other methods to use as well (such as something like this link). You didn't mention the version of Inventor you're using so hopefully you can open this Inventor 2010 version (that's the oldest version I have installed).
You might also see JDMather's knot tutorial for more on the general technique:
I hope this helps.
Best of luck to you in all of your Inventor pursuits,
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