Create a before business rule on table sc_request with below details.
Table: sc_request
When to run: Before
Insert: Checked
Advanced > Condition : Empty
Script:
var grRITM = new GlideRecord('sc_req_item');
grRITM.addQuery('request', current.sys_id);
grRITM.query();
while (grRITM.next()) {
if (!JSUtil.nil(grRITM.variables.requested_for)) { //assuming requested_for is the name of your catalog variable. Replace this exactly with the name of catalog variable you have.
current.requested_for = grRITM.variables.requested_for; /assuming requested_for is the name of your catalog variable. Replace this exactly with the name of catalog variable you have.
}
