Check checkboxes according to google form checkbox

[Crystal Tan]

Hi, I've created a Google Form that has checkbox options that are identical to the one of the Jira ticket's custom field, and using Apps Script to integrate with Jira. The checked values would be saved to a Google Sheet, with ", " separating them. So I've tried

var arrayofval = last_row_values[6].split(", ");

to get an array of selected value. Then use

"customfield_id": arrayofval,

However, it's giving me an error: 

The <field name> value must be an object

If I use 

var arrayofval = last_row_values[6];

there would be an error of

Specify the value for<field name> in an array

Does anyone know how to solve this?