Qrew LegendYou say that you can set the Key field of the Parent table to be the Tax ID. So I suggest you go ahead and do that.
Create that relationship where one parent has many children and select the tax ID field on the child record for the reference field which will end up being on the right side of the relationship.
Once you do this when creating a child record launched off a parent, the ad child record should automatically fill in the field for tax ID, and conversely if your workflow is that the user enters the child record first, then there should be a nice drop-down list of tax IDs for the user to choose from, and of course you can control which fields are presented to the user in the drop-down list to help them to choose the correct Parent. There are various ways to control those fields but the basic one is to go to the advanced properties of the parent table and select up to three fields as the most relevant record picker fields.
