The value you store in your component property multiSelect should be an array of the selectable values you want to be checked:

For example (following your data set):

this.multiSelect = ['x-b', 'rt-builder', 'max']

Whereas for “simple” select fields (single choice)

<q-select ... v-model="selectedValue" :options="options" />

you simply do

this.selectedValue = 'identifier'

Answered here too:
https://stackoverflow.com/questions/47035330/editing-the-selected-option-in-the-select-dropdownq-select-of-quasar-framewor/48227687#48227687