how to create Custom Attribute for RTC with multiple values

How to create custom attribute which stores multiple value?

I need to create a custom attribute with enumeration( list of values).
I want to get more than one selected as value and store the mutiple value in rtc

I read someone comments as follows:

"You need to add the enumeration then create a new attribute and set it to string. When you add the presentation select the attribute and set Kind to Multi-Select List. In the properties add the key "enumeration" and set the value to the enumeration ID you originally set up "

However, I found there is no choice as Multi-Select List in the presentation when I create a new attribute and set it to string.

Any suggestions on how to resolve it? Thanks!