Transform this particular line:
accntList = List.Distinct(accnt[Id]),
code will look like this:
let
Source = Salesforce.Data("https://login.salesforce.com/", [CreateNavigationProperties=true]),
MyCustomObject = Source{[Name="MyCustomObject "]}[Data],
accnt = Source{[Name="Account"]}[Data],
accntList = List.Distinct(accnt[Id]),
#"Filtered" = Table.SelectRows(MyCustomObject, each List.Contains(accntList, [AccountId__c]))
in
#"Filtered"
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