I hope that user name is saved in tinydb.
So
set project bucket to Usuarios/tinydb value
Call firebase tag user name
Call firebase tag password
When firebase got value
If get tag = user name
If get value = text box value
Then (If get tag = password(if get value= user password text, ) then ~imagine all are correct if this point trigger))else alert wrong password
Else alert wrong user name
Or pls ref this AIA Displaying and Storing User Data in Firebase - #36 by Spicy_Topics used same logic
And see the blocks Displaying and Storing User Data in Firebase - #28 by Spicy_Topics at here