I did more research and essentially what I have to do in B4J is just to add the following:
B4X:
srvr.AddHandler("/webhook", "Webhook", False)
Then I create a new class module called Webhook doing things based on the specific method/headers/payload that the calling service will generate and sending a response as the calling service expects to acknowledge I have received the call.
B4X:
'Class module
Sub Class_Globals
Private mreq As ServletRequest 'ignore
Private mresp As ServletResponse 'ignore
End Sub
Public Sub Initialize
End Sub
Sub Handle(req As ServletRequest, resp As ServletResponse)
mreq = req
mresp = resp
Dim method As String
Dim response_type As String
Dim clientid As String
method = req.Method 'Collect the Http method
Log(method & " " & req.FullRequestURI)
clientid = req.GetHeader("x-tropical-clientid")
Log("x-tropical-clientid: " & clientid)
response_type = req.GetParameter("response_type")
Log("response_type: " & response_type)
resp.SetHeader("x-tropical-clientid", clientid)
resp.Write($"{"message":"Hello"}"$)
End Sub