Translate Excel Formula into Power Query

I would like to add a column with "Account No" in the General Ledger extracted from Yardi. Please see an example screenshot below:

Essentially, I need to fill in the column "Account No" with the values in column A that start with "11" for each row that has data. In the example screenshot, this would be only in rows 19 and 20.

I accomplished this with an Excel formula in column "L":

=IF(ISBLANK(B19);"";IF(ISNUMBER(LEFT(L18;1)*1);L18;A18))

I would need to translate this formula (or equivalent) into a Power Query custom column, but I have not idea how to do that.

Can you please help me with this?

Since you chose to provide only a screen shot, and not text which could be easily copy/pasted, my solution uses only the relevant columns.

You will need to adapt the code to include your other columns.

Algorithm:

• Group by the Property rows that start with "1" using the optional fourth and fifth arguments of the Table.Group function.
• Then apply your formula algorithm to each subgroup

Source Data:

M-Code from Advanced Editor
Edit: Code altered to test that Property starts with any digit

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"Property", type text}, {"Property Name", type text}, {"Account No", type any}}),

    x = Text.StartsWith("1!","1"),
    
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Property"}, {
        {"AcctNum", (t)=>Table.ReplaceValue(
            t,
            each [Property Name],
            t[Property]{0},
            (x,y,z)=> if y=null then null
                        else if x <>null then x
                        else z,
            {"Account No"}), 
            type table [Property=nullable text, Property Name=nullable text, Account No=nullable text]}},
         GroupKind.Local,(x,y)=>Number.From(
             List.Contains({"0".."9"}, Text.Start(x[Property],1)) and 
             List.Contains({"0".."9"}, Text.Start(y[Property],1)))),
    
    
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Property"}),
    
    #"Expanded AcctNum" = Table.ExpandTableColumn(#"Removed Columns", "AcctNum", {"Property","Property Name", "Account No"})
in 
    #"Expanded AcctNum"

Result: