1. Pre-define SLAs with different ETAs.
2. Go to Admin > Service Catalog > Field & Form Rules > On Feild Change > Set "Selected Templates" and choose the Templates > On Field Change > Set Priority.
3. Under Actions > Execute Script and make use of the below Script:
setTimeout(function(){
var priority = $CS.getText("PRIORITY");
var SLA = "Select";
if (priority === "High") {
SLA = "SLA 3";
} else if (priority == "Medium") {
SLA = "SLA 1";
}
jQuery('[id^=servicesla_option_]:contains("' + SLA + '")').trigger("click");
},400);