How to hide a multi-select picklist field in a web form?

When you create web forms using CRM, you have an option to hide any field except the ones marked as required field.

To hide a multi-select picklist field in a web form, follow the steps given below. (The procedure to hide a field is the same regardless of the type of the field.)

Go to Setup > Website Integration > Web Forms.
Select the web form you want to edit and in the WYSIWYG Editor, drag and drop the desired picklist field. Let's say there is a field here called Interests.
Click the field's Settings icon.
Select the Mark as Hidden Field checkbox and select a default value. Click Done

When the form is published, your website visitor will not be able to see this picklist field. But when they fill and submit the form, the default value you have specified will be recorded in CRM along with other details.