Create the LedgerDimension RecId from Mainaccount and the default dimension RecId using X++ in D365FO

 LedgerDimensionFacade::serviceCreateLedgerDimension

(LedgerDefaultAccountHelper::getDefaultAccountFromMainAccountRecId

(MainAccount::findByMainAccountId(_mainAccountId).RecId),

defaultDimensionRecId);

Main account and financial dimensions combination should be configuared

How to get MainAccount using ledger dimension recId in ax 2012

Table.MainAccountId             =   MainAccount::findByLedgerDimension(ledgerDimension).MainAccountId;