To create a brand new airport, click anywhere on the airport grounds, at the location where you want your aircraft to spawn. Next, enter the appropriate ICAO code if there is one as your waypoint ID, the name of the airport as your description and select whatever altitude (elevation) you want. If you’re not certain of the elevation, just leave it at 0 and Microsoft Flight Simulator will spawn your aircraft at ground level.
*Note: In most cases the differences in altitude won't cause a problem, you might notice your aircraft "dropping" onto the runway occasionally. But if the airport has a significant slope (in particular mountainside airstrips) it's possilbe the landing gear, or even the entire aircraft will spawn in the ground.
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