This is currently not possible due to a limitation of the conduit API. A possible solution would be to use the sprint.create method from the sprint extension but that would create a dependency on the sprint extension.
If we decide to use sprint.create we should at least fall back gracefully to project.create if the sprint extension is not installed on the target Phabricator.

Technically, the "is sprint" field is added by the Sprint Extension, and is not a part of phabricator itself. My understanding is that the Sprint Extension could extend the conduit API to expose this field.

This is currently not possible due to a limitation of the conduit API. A possible solution would be to use the sprint.create method from the sprint extension but that would create a dependency on the sprint extension.
If we decide to use sprint.create we should at least fall back gracefully to project.create if the sprint extension is not installed on the target Phabricator.

@Tobi_WMDE_SW: I thought Phragile already depended on the sprint extension, in that without it, no tasks would have story point estimates.

@ksmith Phragile needs a custom Maniphest field for story point estimates but it does not matter whether that field gets added manually through the admin interface or through the sprint extension.

See T106816. This is done.