Like they say in the olden days, Linux - eh Splunk, can do anything except brew coffee. Can you qualify your requirement? Is the time range from a dashboard's data input of type Time? In that case, starttime and endtime are in the token name that you give the input. If you want a specific presentation of those values in a search, you just use the likes of strftime to manipulate them.
If you want specific help, you need to clearly state your use case including desired output. If you want to use one selector to set values in other selectors as your mock screenshot seems to suggest, that is doable, too. But you need to describe the desired behavior in unmistakable detail.
