At the moment, it’s not supported to retrieve the values from a multi-select field when writing formulas. You’ll need to change the structure to a subtable and let users input one option in each line. Then, apply formulas from there.
I have to be honest.
That makes this filed type mostly unusable except for display purposes, as you can not use it with logic, reporting or in code. I would suggest warning users not to use this field if they need to be able to perform any functions on the field values.
Making a sub-table is work around and will not always be practical as it makes the forms longer.
I did some testing with this. You can javascript to getFieldValue() of a multi-select field. It will be returned as string with commas separating the values. To set the field value, you need to enter the values as one string with a pipe separator
so like this: record.setFieldValue(multiField,“selectionOne|selectionTwo”);