Effect of prestress on eigenfrequency

Hello,

I am trying to model the effect of the prestress on eigenfrequency of a vibrating string (clamped at both end with a prestress)
The problem is that I am using 3.4 here. Would you please let me know how to load a static analysis result into a eigenfrequency analysis?
What should I do to introduce stress to the material?
Should I use matlab to load the prestress?
Is there a simple way to do that?
Is there simply a way to do it?

I am not able to find good exemples corresponding to that problem with my COMSOL version (3.4).

What should I do to introduce stress into the material?

Thanks.