How can I optimize this query for better performance

As olafhelper said, the UDF is the source of your issue. There are 2 ways to modify your query

1. Use JOIN and aggregated function in a subquery/CTE, then JOIN with p_Ledger to get result.
2. Use CROSS APPLY replace the UDF, so the query might run in parallel.

Code 1

WITH CTE
AS
(
SELECT 
	T.CompanyId, T.AccountsId, T.branchId, L.CreationDate,
    SUM(IIF(T.transactionStatusId = 1, T.amount, -T.amount)) [FirstBalance]
FROM p_Ledger L
INNER JOIN dbo.Transactions T 
ON
    T.CompanyId = L.CompanyId
    AND T.AccountsId = L.AccountId
    AND T.transactionsDate < L.CreationDate
    AND T.branchId = L.BranchId
WHERE 
	L.CreationDate >= '2024-11-01'
    AND L.CreationDate <= '2025-02-11'
    AND L.CompanyId = 117
    AND L.branchId = 161
	AND T.Approval = 1 
    AND T.IsDeleted = 0
GROUP BY
	T.CompanyId, T.AccountsId, T.branchId, L.CreationDate
)
SELECT
	ROW_NUMBER() OVER (ORDER BY L.CreationDate DESC) AS RowId,
    L.Id [Id],
    L.transactionsId [TransactionsId],
    L.amount [Amount],
    L.AccountId [AccountId],
	ROUND(ISNULL(CTE.FirstBalance, 0), 2) [FirstBalance]
FROM p_Ledger L LEFT OUTER JOIN CTE
ON L.AccountId = CTE.AccountsId
AND L.CompanyId = CTE.CompanyId
AND L.branchId = CTE.branchId
AND L.CreationDate = CTE.CreationDate
WHERE L.CreationDate BETWEEN '2024-11-01' AND '2025-02-11'

Code 2

SELECT 
    ROW_NUMBER() OVER (ORDER BY CreationDate DESC) AS RowId,
    Id AS [Id],
    transactionsId AS [TransactionsId],
    amount AS [Amount],
    AccountId AS [AccountId],
    ROUND(COALESCE(calc.FirstBalance, 0), 2) AS [FirstBalance]
FROM p_Ledger L
CROSS APPLY (
    SELECT 
        SUM(CASE WHEN T.transactionStatusId = 1 THEN T.amount ELSE -T.amount END) AS FirstBalance
    FROM dbo.Transactions T
    WHERE 
        T.Approval = 1
        AND T.CompanyId = L.CompanyId
        AND T.AccountsId = L.AccountId
        AND T.IsDeleted = 0
        AND T.transactionsDate < L.CreationDate
        AND T.branchId = L.branchId
) calc
WHERE 
    L.CreationDate >= '2024-11-01'
    AND L.CreationDate <= '2025-02-11'
    AND L.CompanyId = 117
    AND L.branchId = 161

NOTE

You should do some test to confirm which way is the best, and if they will get result correctly.