how to add onchanget event to select field

Hello, I would like to add javascript 'onChange' event handling for a select field, such as     <div class="form_row">         <label for="s_modify">Modifiers</label>         <%= form.select :s_modify, ([ "Frequency", "Genotype", "Germline", "Group", "Haplotype", , "Metagenomic", "Pathovar", "Plasmid-name", "Plastid-name", "Pop-variant" , "Rearranged", "Segment", "Subclone", "Subgroup", "Substrain", "Subtype", "Synonym", "Transgenic", "Type", "Variety" ]) %>       <label for="vaule_sm">Value</label>        <%= form.text_field :vaule_sm%>      </div>

by if user select Metagenomic or 'Germline or 'Rearranged' or 'Transgeni.

the vaule_sm is "TRUE"

Anyone know how to make this work?