How do I multiply a search result with dynamic value entered through a form?

I have a value called total produced by this search: index="_internal" source=*license_usage.log type=Usage st($st$) | bin _time span=1d | eval KB=round(b/1024) | eval MB=round(KB/1024,2) | eval GB=round(MB/1024,2) | eval TB=round(GB/1024,2) |stats sum(MB) as Total I want to multiply it with a dynamic value entered by a user through a form. How do I do that?