https://myinstance.service-now.com/incident.do?sys_id=-1&sysparm_query=customer=123^issue=sitedown^urgency=3

Note that in the sysparm_query part you need the actual field name, not its label (e.g., "u_my_field" vs. "My Field"), and you need to use values vs. labels for choice list fields (like "3" vs "Low" for the urgency field.