How can I enable multiple values after initialization?

Good morning!

I have a form which have several TomSelect Control and my aim is conditionally allow multiple values in one TomSelect control.

In one case it allows only one value and in another case it allows multiple values.

A “normal” SelectBox can be edited by JavaScript

selBox.setAttribute("multiple", true);

But I doesn’t work on TomSelect. How can I solve this idea?

selBox.setAttribute("multiple", true);

Thanks in advance,

Axel