The URL syntax is:
https://outlook.office365.com/api/v1.0/me/folders/<FOLDER ID>/Messages
So you need to get the ID for the folder you want to query. For example, if it's a subfolder of the Inbox, you could do a GET to:
https://outlook.office365.com/api/v1.0/me/folders/inbox/childfolders
And you'd get back something like:
{
"@odata.context": "https://outlook.office365.com/api/v1.0/$metadata#Me/Folders('inbox')/ChildFolders",
"value": [
{
"@odata.id": "https://outlook.office365.com/api/v1.0/Users('JasonJ@contoso.com')/Folders('AAMkADNhMjcxM2U5LWY2MmItNDRjYy05YzgwLWQwY2FmMTU1MjViOAAuAAAAAAC_IsPnAGUWR4fYhDeYtiNFAQCDgDrpyW-uTL4a3VuSIF6OAAAeY0W3AAA=')",
"Id": "AAMkADNhMjcxM2U5LWY2MmItNDRjYy05YzgwLWQwY2FmMTU1MjViOAAuAAAAAAC_IsPnAGUWR4fYhDeYtiNFAQCDgDrpyW-uTL4a3VuSIF6OAAAeY0W3AAA=",
"ParentFolderId": "AAMkADNhMjcxM2U5LWY2MmItNDRjYy05YzgwLWQwY2FmMTU1MjViOAAuAAAAAAC_IsPnAGUWR4fYhDeYtiNFAQCDgDrpyW-uTL4a3VuSIF6OAAAAAAEMAAA=",
"DisplayName": "New Subfolder",
"ChildFolderCount": 0
}
]
}
Then take the value of the Id
field and plug it into the URL:
https://outlook.office365.com/api/v1.0/me/folders/AAMkADNhMjcxM2U5LWY2MmItNDRjYy05YzgwLWQwY2FmMTU1MjViOAAuAAAAAAC_IsPnAGUWR4fYhDeYtiNFAQCDgDrpyW-uTL4a3VuSIF6OAAAeY0W3AAA=/Messages