I think if you place a marker where you want the roadblock, then place a custom objective to only use that marker as its TAOR, you can then select a roadblock from the dropdown in the module. Don't quote me on that as I'm not sure if a roadblock is a composition you can select in that list.
If not, try placing the same marker, then a civilian objective module using that marker as its TAOR (blacklist the rest of the map). Then set roadblocks to high, give or take. From there try seeing if setting it to objectives only within the module works, or if not, place an infantry of 1 (0's in the rest of the lines).
While you're in there, also check if roadblocks are a composition you can select within the civilian objective module (not home and have never looked). If so, that's probably the easiest way (that, or selecting the same from the custom objective module instead if it's available as a choice).