You may set a region for placement and routing but not a specific routing path. To lock the region, you may use logic lock.
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ug/ug-qpp-design-optimizati... Chapter 6.5.1. Creating Logic Lock Assignments with Tcl commands
You can create the Logic Lock region using the GUI or qsf assignment.
1) use the assignment below to create a new placement region with bounding box
coordinates X46 Y36 X65 Y49:
set_instance_assignment -name PLACE_REGION "X46 Y36 X65 Y49" -to <node names>
2) use the assignment below to create a routing region with bounding box coordinates X46 Y36 X65 Y49: set_instance_assignment -name ROUTE_REGION -to "X46 Y36 X65 Y49"
Please note that all instances with a routing region assignment must have a respective placement region; the routing region must fully contain the placement region.