PowerBI DAX: Formula for an "Initial Transaction Flag"

Question

So imagine I have a table "Transactions"

with these columns:

ColumnName	DataType
TrnID	ID
Customer	String
Book Date	DateTime
Amount	Float
Campaign	String

And the following Rows

TrnID	Customer	Book Date	Amount	Campaign	Expected	Measure	Column
1	Albert	03-24-23	100	A	true	false	true
2	Albert	03-27-23	50	A	false	false	true
3	Albert	03-29-23	25	A	false	false	false
4	Suzi	03-27-23	50	A	true	true	true
5	Boris	03-27-23	500	A	true	true	true
6	Martha	03-27-23	10	A	true	true	true
7	Anna	03-21-23	50	A	true	false	true
8	Anna	03-22-23	50	A	false	false	true
9	Anna	03-27-23	5	A	false	false	false

QUESTION: How would I get an information in each row, if the transaction is an initial transaction?

That means from a set of transactions with the same [Customer] and the same [Campaign], it is the transaction with the lowest [Book Date].

Unfortunately my experience is limited yet.

ATTEMPT 1) I tried a measure:

InitialTransaction  =

VAR InitialTransaction =
    CALCULATE(
        MIN(Transaction[BookDateMEZ]),
        ALLEXCEPT(Transaction, Transaction[Customer], Transaction[Campaign])
    )
RETURN
    IF(SELECTEDVALUE(Transaction[Book Date]) = InitialTransaction, TRUE(), FALSE())

OUTCOME:

1. see above in the column "Measure"

ATTEMPT 2) I tried a Clolumn:

InitialTransactionColumn  =

VAR TransactionRank =
    RANKX(
        FILTER(
            Transaction,
            Transaction[Customer] = EARLIER(Transaction[Customer]) &&
            Transaction[Campaign] = EARLIER(Transaction[Campaign])
        ),
        Transaction[Book Date],
        ,
        ASC,
        DENSE
    )
RETURN
    IF(TransactionRank = 1, TRUE(), FALSE())

OUTCOME:

1. see above in the column "Column"
2. The Arguments of the EARLIER-function are underlined in red
3. Despite 2. the DAX editor throws no error and the visual doesn't break.

Answer 1

you can try this t create a column

Column 2 = 
VAR _min =
    MINX (
        FILTER (
            'Table (2)',
            'Table (2)'[Customer] = EARLIER ( 'Table (2)'[Customer] )
                && 'Table (2)'[Campaign] = EARLIER ( 'Table (2)'[Campaign] )
        ),
        'Table (2)'[Book Date]
    )
RETURN
    IF ( 'Table (2)'[Book Date] = _min, TRUE (), FALSE () )

or try this to create a measure

MEASURE 2 =
VAR _min =
    CALCULATE (
        MIN ( 'Table (2)'[Book Date] ),
        ALLEXCEPT ( 'Table (2)', 'Table (2)'[Campaign], 'Table (2)'[Customer] )
    )
RETURN
    IF ( _min = MAX ( 'Table (2)'[Book Date] ), TRUE (), FALSE () )