In BUSY you can mannually create Tax category for services,in case of service you can mention in item master tax category of service and in case of goods you can mention in item master tax category of goods, to create tax category,you can follow these steps:
Step 1:Go To Administration menu,
Step 2:Select Masters,
Step 3:Click Tax category Master,Then click add,Mention the Tax category name here service 18% as you want,then select Type as service,mention GST Tax Rate and save and the tax category of goods is by default created in BUSY.
Step 4:Then while creating service item in BUSY Accounitng Software,you will have to select Tax category as service 18% in item master.
Now you can generate e invoice of service item, it will be generated.