Dynamics 365 V9.0 Multiselect option

Get Value from Multi select control

Xrm.Page.getAttribute('new_testmultiselect').getValue()

Set Value from Multi select control

var arr=new Array();
arr.push(100000002);
arr.push(100000001);

Xrm.Page.getAttribute('new_testmultiselect').setValue(arr)
or
Xrm.Page.getAttribute('new_testmultiselect').setValue([100000002,100000001])

Set null/empty Value from Multi select control

Xrm.Page.getAttribute('new_testmultiselect').setValue(null)

Multiselect control is not applicable for the following 

• Set multiselect field values through the business rule
• Set multiselect field values through a workflow