Hello,
out of the box ServiceNow offers different request methods "Order" "Request" and "Submit". However, in this case not only the button label changes, but also the functionality behind. E.g. the "Order" type will include a basket, while submit will directly submit the form.
If I understand your question correctly, you just want to rename the button to "submit", but keep the "Order Now" functionalirty. In this case, I would suggest the following thread in which there are different options shown on how to rename the "Order Now" button: https://community.servicenow.com/community?id=community_question&sys_id=431343e1dbd8dbc01dcaf3231f961971
I hope this helped!
Greetings,
Sören
