Hanging a large mass on the end of the string will provide any tension you want. The string can obviously support it because that is the tension in normal operation. A mass (several kg) on a pulley is a lot cheaper than using any form of force meter for that sort of tension. It's a fairly common experiment. The way I have always excited the string has been to pass a current through the string and to put a fat magnet right next to it near an antinode. It achieves a very light coupling which doesn't affect the measurement.
As the formula involves three variables, you have to know two of them in order to calculate the third - (frequency).
You need to clamp the whole thing firmly or the mass will pull the whole thing over.