Hey,
I think you mean a vertical line. If you are asking about an underscore so that the user can do a hot key combination, that is not available when creating a toolbar in the manifest.
The only way I have been able to simulate a vetical line is to use the pipe character (|) (see the second button capture.:
<xsf:toolbar caption="PopInfo" name="PopDataFields">
<xsf:button caption="Show Sample Data" name="PopDataFields" tooltip="Show Sample Data"></xsf:button>
<xsf:button caption="| Remove Sample Data" name="ClearDataFields" tooltip="Remove Sample Data"></xsf:button>
</xsf:toolbar>
Check out the SDK for the Button, Toolbar, Menu and MenuArea element.
You might have to do a "__________" for the horizontal spacer like I used the "|" as a vertical spacer on the toolbar.
Hope that helps,
Mary