get warning message in one table based on the info in anther table

Hi all

I have 2 tables ( A,B), and i am struggling to work out the below:

People can enter many records in Table B, each record has got 1 field ( ID Number) which represent a customer with a complaint.

Then

If the same ID number entered in field X in Table A i want to show a warning message that this customer has got a complaint ( based on the data available in Table B). I would normally apply the formula If(Contains(field D, Field X)...... but not sure how to pull the data from Table B to field D? or maybe there is a better way?

When i created a relationship between  the two tables the values from table B were showing as a dropdown list in table A which is not helpful if i want to apply If(Contains.. 
any ideas?

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Eyad Hassoun
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