form code.bas

Private Sub Cancel_Click()
Me.Cancel = True
Unload Me
End Sub

Private Sub OK_Click()
Unload Me
End Sub

Send email from Outlook to Joplin.bas

Public Sub SendToJoplin()
    Dim sToken As String, sURL As String
    Dim sURLNotes, sURLResources, sEscapedBody, sJSONString, sFolderID As String
    Dim objItem As Outlook.MailItem
    
    sToken = "REPLACE WITH YOUR TOKEN"
    sURL = "http://127.0.0.1:41184"
    sURLNotes = sURL & "/notes?token=" & sToken
    sURLResources = sURL & "/resources?token=" & sToken

    For Each objItem In ActiveExplorer.Selection
        sEscapedBody = EscapeBody( _
                "Date: " & objItem.ReceivedTime & "<br>" _
                & "From: " & objItem.Sender & "<br>" _
                & "To: " & objItem.To & "<br>" _
                & "CC: " & objItem.CC & "<br>" _
                & "BCC: " & objItem.BCC & "<br>" _
                & objItem.HTMLBody)
        
        sFolderID = GetFoldersFromJoplin(sToken, sURL)
        If IsEmpty(sFolderID) Then
            Exit Sub
        End If
                        
        With CreateObject("MSXML2.XMLHTTP")
            .Open "POST", sURLNotes, False
            .Send "{ ""title"": """ & objItem.ConversationTopic & """" _
            & ", ""parent_id"": """ & sFolderID & """" _
            & ", ""body_html"": """ & sEscapedBody & """" _
            & " }"
            Do Until .readyState = 4: DoEvents: Loop
                sJSONString = .responseText
        End With
    Next
    'Debug.Print sJSONString 'Uncomment to see joplin response
End Sub

Private Function EscapeBody(sText As String)
    EscapeBody = sText
    EscapeBody = Replace(EscapeBody, "\", "\\")                 'Backslash is replaced with \\
    EscapeBody = Replace(EscapeBody, Chr(34), "\" & Chr(34))    'Double quote is replaced with \"
    EscapeBody = Replace(EscapeBody, vbCr, "\r")                'Carriage return is replaced with \r
    EscapeBody = Replace(EscapeBody, vbLf, "\n")                'Newline is replaced with \n
    EscapeBody = Replace(EscapeBody, Chr(8), "\b")              'Backspace is replaced with \b
    EscapeBody = Replace(EscapeBody, Chr(12), "\f")             'Form feed is replaced with \f
    EscapeBody = Replace(EscapeBody, vbTab, "\t")               'Tab is replaced with \t
End Function

Private Function GetFoldersFromJoplin(sToken As String, sURL As String)
    'Input token, url
    'Output folder id
    
    Dim sJSONString, sMyChoice As String
    Dim vJSON As Variant
    Dim sState As String
    Dim aData(), aHeader()
    Dim i, hieght As Integer
    Dim OpBtn(20) As MSForms.OptionButton

    sURL = sURL & "/folders?token=" & sToken
    
    'Get folders list
    With CreateObject("MSXML2.XMLHTTP")
        .Open "GET", sURL, False
        .Send
        Do Until .readyState = 4: DoEvents: Loop
            sJSONString = .responseText
    End With
        
    'Parse JSON response
    JSON.Parse sJSONString, vJSON, sState
    JSON.ToArray vJSON, aData(), aHeader()
    
    'Display a choices of folders
    height = 100
    For i = LBound(aData) To UBound(aData)
        'Add Dynamic OptionButton and assign it to object 'OpBtn'
        Set OpBtn(i) = fChooseNoteboo.Controls.Add("Forms.OptionButton.1")
        With OpBtn(i)
            .Caption = aData(i, 2)
            .Name = "oNotebook" & i
            .Top = 25 + i * 25
            .Left = 25
        End With
        height = height + 25
    Next i

    fChooseNoteboo.height = height
    fChooseNoteboo.OK.Top = height - 60
    fChooseNoteboo.Cancel.Top = height - 60
    fChooseNoteboo.Show
    
    If fChooseNoteboo.Cancel.Cancel = True Then
        Exit Function
    End If
    
    For i = LBound(aData) To UBound(aData)
        If OpBtn(i).Value = "True" Then
            GetFoldersFromJoplin = aData(i, o)
            Exit For
        End If
    Next i
End Function

Any progress on this? I wasn't able to get it installed as a Macro on Outlook 365, the fChooseNoteboo.OK.Top = height - 60 is not found according to the debugger.

I stopped using Joplin. You're free to play with the code if you wish.