Assign tasks to user selected in a custom field

Assign tasks to user selected in a custom field

I would like to assign a task to a user selected in a custom field, when the deal stage changes. I think I could do it with a custom function (I don't have experience with custom functions). Is this possible? To be clear, there are two users related to each deal, record owner and Sales Support. I know how to assign a task to the record owner, but I would like to assign a task to the Sales Support user. Sales Support user is different for each deal, depending on other factors.