The following snippet allows you to add a Select field to a Frontend Form that works with Content REST API.
var fieldValue = $('#yourFieldVelocityVarName').val(),
## Declare global path of dojo libraries
#set($dojoPath = $webapi.getConfigVar("path.to.dojo"))
#if(!$UtilMethods.isSet($dojoPath))
#set($dojoPath = "/html/js/dojo/1.4.0")
#end
## Call required dojo libraries for FilteringSelect fields
<script type="text/javascript">
dojo.require("dojo.parser");
dojo.require("dijit.form.FilteringSelect");
</script>
## Code of your Select Field
#set($contentTypeVelVarName = 'yourContentType')
#set($selectFieldVelVarName = 'yourSelectField')
#set($contentType = $structures.findStructure($contentTypeVelVarName))
#set($fields = $structures.getFields($contentType))
#foreach($fieldAux in $fields)
#if($fieldAux.fieldType == 'select' && $fieldAux.velocityVarName.equals($selectFieldVelVarName ))
<label for="countrySelector">Country Selector</label>
<select dojoType="dijit.form.FilteringSelect" id="$fieldAux.velocityVarName" name="$fieldAux.velocityVarName" autoComplete="false" class="selectField">
#set($valuesSplit = $fieldAux.values.split("\n"))
<option value="" class="optionSelectField">Select an option</option>
#foreach($value in $valuesSplit)
#set($labelValue = $value.split('\|'))
#set($label = $listTool.get($labelValue, 0).trim())
#if($listTool.size($value.split("|")) > 1)
#set($value = $listTool.get($labelValue, 1).trim())
#else
#set($value = $listTool.get($labelValue, 0).trim())
#end
<option id="${fieldAux.velocityVarName}$value" value="$value" class="optionSelectField">$!label </option>
#end
</select><span id="alert${fieldAux.velocityVarName}" class="errorMessages"></span>
#end
## End of Select field