Assuming you are using CGI, you'll want to follow the sample code for scrolling lists, with particular attention to using the values and labels options to relate an array of values to a hash of label/value pairs.

Assuming you are simply asking how to get the '1340' out of the 'LN_TH_13.40.98_0010', then you will probably want to apply a regular expression of some kind or split according to some character (like '_' and '.'). Show us what you've tried and we'll be more likely to help.

Seriously, though, davorg's advice is sound ... we really have no idea what you are asking. Recommend you update your question with a clearer explanation of what you are attempting and what you've tried.