Connect to a Web Service sending parameters

This is a function that takes a token as a mandatory parameter and a path as an optional one to fetch data from dropbox for example. If this doesn't help, we would need more details about where your exact problems lie:

let Source=
(token as text, optional folder as text) => 
let
    data = [    path= if folder = null then "" else folder,
                recursive=false,
                include_media_info=false,
                include_deleted=false,
                include_has_explicit_shared_members=false
],
    header = [  #"Authorization"="Bearer "&token,
                #"Content-Type"= "application/json"],
    response = Web.Contents("https://api.dropboxapi.com/2/files/list_folder",[Content=Json.FromValue(data),Headers=header]),
    out = Json.Document(response,1252),
    entries = out[entries],
    ToTable = Table.FromList(entries, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    ExpandProperties = Table.ExpandRecordColumn(ToTable, "Column1", {".tag", "name", "path_lower", "path_display", "id", "client_modified", "server_modified", "rev", "size", "content_hash"}, {".tag", "name", "path_lower", "path_display", "id", "client_modified", "server_modified", "rev", "size", "content_hash"}),
    RetrieveContent = Table.AddColumn(ExpandProperties, "File", each Web.Contents("https://content.dropboxapi.com/2/files/download",[
    Headers=[#"Dropbox-API-Arg"="{""path"":"""&[path_display]&"""}", #"Authorization" = header[Authorization]]]))

in
RetrieveContent 

,documentation = [	
Documentation.Name =  "	fnDropbox.Folder
", Documentation.Description = "	Returns a table with contents from your selected Dropbox folder
" , Documentation.LongDescription = "	Returns a table with contents from your selected Dropbox folder. The optional field 'folder' allows you to access sub-folders within the main folder.
", Documentation.Category = "	Accessing data functions
", Documentation.Source = "	local
", Documentation.Author = "	Imke Feldmann: www.TheBIccountant.com
", Documentation.Examples = {[Description =  "	
" , Code = "	Check this blogpost explaining how it works: http://wp.me/p6lgsG-AA
 ", Result = "	
"]}]	
 in	
  Value.ReplaceType(Source, Value.ReplaceMetadata(Value.Type(Source), documentation))