On desktop browsers, the default multi-select dropdown in HTML5 requires you to use the CTRL and SHIFT modifier keys to multi-select, which is not idea for most web users.
Adrienn Fehér shared a javascript-enhanced version that works quite smoothly ( I've not tested on mobile ).
https://codepen.io/sa-js/pen/ZwXPbR
Collecting the values as CSV
Further she added a bit to show how to submit the selected items as comma-separated values. The main use case? When you want to be able to choose which mailing list groups you want to subscribe to on MailChimp.
// get the form-select's selected value
const selectVal = $('#IdOfYourSelectField option:selected')
.toArray().map(item => item.text).join();
Finsweet approach
Finsweet added a library that allows you to use Webflow's stylable menu dropdown elements to create your form selects.
https://finsweet.com/attributes/custom-form-select
