When you say "tensioned fabric" I assume you're concerned about 2nd order fabric drape effects in the same way that folks working with cables are concerned about the 2nd order cable sag effects.

Is that correct? If so, then RISA is not an ideal program for that type of application. Just like it would not be an ideal application for a suspension cable bridge like the golden gate.

Now, you can still model these types of structures relatively easily. The quality of your analysis results will just depend on the degree of 2nd order effects. If you're expecting a large amount of 2nd order effects then your results will not be all that good. If you are happy with 1st order effects then you should be satisfied with the analysis results RISA gives.

I am not really too worried about 2nd order effects. I am just curious how to model the tensioned fabric. How do I input the tension? Does it need to be applied externally? Is there any sort of reference for material properties?

You need to have the reference for the material properties.

Regarding inputting fabric tension: You could always apply a negative thermal load. That's using a plate surface load with the direction = T (for temperature). Then you can adjust the level of thermal force to get yourself some pre-tension. Basically the same procedure that we talk about in the section in the help file on thermal loads and pre-tensioning cables.