Hello CRMQQ!
In order to set the values for a Multiselect option field, you have two ways:
1. Using Microsoft Dataverse Web API:
// On the parameter object, you add the following property:
{
"new_multiselectField": "1, 9"
// 1 and 9 are the values of the options you want to select, if you just want one, simply add the value of that option
}
2. Using formContext
//You create an array containing the values of the options you want to select
formContext.getAttribute("new_multiselectField").setValue([100000003, 100000004]);
Here you also have two guides with more information about the topic: Option 1 (using Web API), Option 2 (using formContext)