To use Join, it is much better to use the syntax instead of the methods, since the Join () method has {4.5} parameters, which makes reading more difficult.
Still, answering your question, your query would look like:
fornecedores.Join(contas, f => f.ID, c => c.FornecedorID, (f, c) => f);
Parameters:
1st: Collection you will Join
2nd: Key to the first collection
3rd: Key to the second collection
4º: Function that returns the result item, it is almost the same as using the Select (), however it receives two parameters, the first is the item from the first collection (suppliers), the second is the item from the second collection ( accounts).