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what method are you using to set the values?

UpdateProfile method in commerce api methods

are you passing an array of internal id's for the multiselect field?

var customerUpdate={};
			
var custdata={custentity_preferred_equip:par_pref};
			customerUpdate.customfields = custdata;
			customer.updateProfile(customerUpdate);

Where par_pref is array of numbers like ("1", "2")

In this way we are passing the values to updateProfile method,but it is getting issue

<@U8X7E86M6>  do we have any reference for setting the multiple values to dropdown in updateProfile method in account.model

do you need to pass the customer internal id?
var customerUpdate = {
	  internalid: parseInt(nlapiGetUser(), 10),
};
var custdata={custentity_preferred_equip:par_pref};
            customerUpdate.customfields = custdata;
            customer.updateProfile(customerUpdate);
ModelsInit.customer.updateProfile(customerUpdate);

Same thing i have done,but i think  iam getting issue in object formation

have you tried using the script debugger?