Add support for multiuser custom field

[capiscuas]

Hi, i have a Validate custom button and I'd like to assign it so that the user that clicks on the button, gets added to a multiuser custom field of that issue called 'Validators'. Problem is that multiuser/even users type custom fields are not filled in by the plugin, any hint? Is there a similar plugin doing this?

https://github.com/Restream/redmine_customize/blob/7b8e08599fe26cb9f4d3d3b7ecf16b1e4928c110/app/views/custom_buttons/_form.html.erb#L97