provided you set a replica for ‘Supplier’ in the Jira OnPrem output script (example replica.supplier), you can in scripting for jcloud in the incoming script do an IF statement to specify the project key it must be created for.
if(replica.supplier?.value == '01') {
// Exalate to Project A
issue.projectKey = 'PROJECTA';
} else if(replica.supplier?.value == '02') {
// Exalate to Project B
issue.projectKey = 'PROJECTB';
}
There may be better ways, but that is how I currently do it.