If you are in-line edit mode bring up the symbol map with Ctrl+M and select the square root symbol. If you are using the Text Control editor enter \g(\(214)) where you want the symbol.
Looks like what you want is a math equation with terms inside the square root. That is not currently supported in Origin.
You could set up the equation/expression say in Microsoft Word and copy-paste that as an image into the graph and use that instead of the axis title. When you copy-paste, you get an EMF object in the graph. You can right-click and select Properties and in the dialog check the "Keep Aspect Ratio" checkbox under the Dimensions tab and then resize the equation and place it where you want on the graph