Create a Global field (text) for the purposes of the search. Assuming the search field is named Date.
Enter your find criteria into the global field in Browse Mode. I like putting it in the header, but you could even use a custom dialog
Script the find:
Enter Find Mode (no pause)
Set Field [Date] Global
Perform find ... Note you can add a lot more to a Find script, based on the results of the find...
Then use the contents of the global field in the email where ever you want.
You didn't say what version of FMP, the above works with the most versions, in 11 you can use a variable.
Yes, and I usually use two global date fields for date ranges. That way I can add drop down calendars and the field validation catches many date typos. I then use an expression in the Set Field expression of a script like Bumper suggests to construct the date range expression by using Table::gDate1 & "..." & Table::gDate2.