Very quick 'IF function' question

If you only have a few accounts to add the percentage, the the formula provided by PwerQueryKees should be sufficient.

If you have a large number of accounts to consider, you would be better off creating a table of accounts vs reponsible percentages which you could then refer to for your Custom column.

Paste the below code into the Advanced Editor to see how that would work. In your real data, you would create the Responsible and Data tables separately.

let

//sample table of responsibility
    #"Responsible Percent" = Table.Buffer(Table.FromRecords({
        [Account="B1234", Percent=0.25],
        [Account="B5678", Percent=0.50]},
        type table[Account=text, Percent=Percentage.Type])),

//sample data table
    Source = Table.FromRecords({
        [Account="B1234"],
        [Account="B5678"],
        [Account="A9876"],
        [Account="A1579"],
        [Account="C1234"]
        },type table[Account=text]),
    
//Add responsible percent column per account
    #"Added Custom" = Table.AddColumn(Source, "Responsible Percentage", 
        each 
            try Table.SelectRows(#"Responsible Percent",(r)=>
                r[Account]=[Account])[Percent]{0} 
            otherwise 0, Percentage.Type)
in
    #"Added Custom"

Responsible Table

Results