ServiceNow provides an example of how to do this using prototype GlideTemplate. One can find it in Script Include global.TestExecutorAjax and its function decodeFieldValues:
// Use GlideTemplate so we can re-use existing code.
var fakeTemplateGR = new GlideRecord("sys_template");
fakeTemplateGR.setValue("template", encodedQuery);
fakeTemplateGR.setValue("next_child",false);
fakeTemplateGR.setValue("table",table);
// GlideTemplate is a Java class used when applying templates from the UI
var glideTemplate = GlideTemplate.getFromRecord(fakeTemplateGR);
glideTemplate.setApplyChildren(false); // n/a for this use case
var targetGR = glideTemplate.apply();
// ...
You could modify it to suite your need, as below (assuming current is the incident in the Business Rule and assuming the names of the fields are u_table and u_set_to):
// Use GlideTemplate so we can re-use existing code.
var fakeTemplateGR = new GlideRecord("sys_template");
fakeTemplateGR.setValue("template", '' + current.u_set_to); // This will add the values selected by the user to the fake temporary template
fakeTemplateGR.setValue("next_child", false);
fakeTemplateGR.setValue("table", '' + current.u_table);
// GlideTemplate is a Java class used when applying templates from the UI
var glideTemplate = GlideTemplate.getFromRecord(fakeTemplateGR);
glideTemplate.setApplyChildren(false);
var newRecord = new GlideRecord('' + current.u_table);
// Begin a new record in the selected table
newRecord.newRecord();
// Apply the values selected to the new record
glideTemplate.apply(newRecord);
// Set other values that must take precedence over the template, like the parent - for instance (assuming table will always be a task):
newRecord.parent = current.getUniqueValue();
// Create the new record
newRecord.insert();
