In the form, use the Add Generic Fields option and use it to add a multi-select field, setting up the options as required.
By default generic field values as discarded once the form submission is complete, but if you want it to be saved to the database you can manually add the "persist":1 option, like so:
Note that if you want to output this field it is a non-Types field, which will be output using the wpv-post-field shortcode instead of the usual Types shortcode.
You could simply add the "multiple" attribute to the select input using JS (see screenshot for an example), but it wouldn't save the results correctly, because a Types Select field expects only one option to be selected.
If you are using a custom field in front-end Forms then a generic multi-select field is your only option that will save multiple options to the database correctly.
i will try a generic field. But is a generic field also visible and usable in the back-end of the ctp or only in the front end?
Why is this so a big problem to have a multi-select field and will some solution come to toolset?
Sorry i am an non dev and therefore this stupid questions?