Morning glory (flower) is classed tender in the cited extension guidance. The timing rules below are relative to your local freeze dates — enter a location to turn them into calendar dates.
The rules this site applies
Method
Frost-relative window
Rule & reconciliation
Direct-sow outdoors
from the median last spring freeze to 2 weeks after
Iowa State University Extension lists morning glory by name among annuals to direct-seed "after the threat of frost is past in the spring", and Colorado State's seed-starting table asterisks morning glory as best suited to direct seeding, placing it two weeks before the frost-free date; the registry's reconciliation policy takes the later, more frost-conservative Iowa State interval for this frost-tender vine.
No start seeds indoors window: No indoor-start window is encoded because Colorado State's asterisk flags morning glory as best suited to direct seeding rather than an indoor start.