AllowedChildTypes is a field of the workspace content, so you can set its value like other field values. The value should be an array of strings (array of content type names).
There's also an OData action for adding a content type to a content's AllowedChildTypes, its name is AddAllowedChildTypes and it should get a contentType array (string array of content type names) as parameter.
dynamic workspace = await Content.LoadAsync(workspace);
Task<dynamic> task = workspace.AddAllowedChildTypes(new {contentTypes = ["File"]});
var result = await task;
AllowedChildTypesis a field of the workspace content, so you can set its value like other field values. The value should be an array of strings (array of content type names).There's also an OData action for adding a content type to a content's AllowedChildTypes, its name is
AddAllowedChildTypesand it should get a contentType array (string array of content type names) as parameter.https://community.sensenet.com/docs/built-in-odata-actions-and-functions/#add-types-to-allowed-child-types-action