How to search for records via Integromat

I am trying to update a table via Integromat/Make and am absolutely stuck. Can someone help?
What I am trying to do:
A web form submits details of a user.

  1. A new record is created in Table A - OK
  2. Next, I want to search Table B to see if the user already exists. If the user already exists in Table B, one column is updated in Table B. If not, a new row/record is created in Table B using the data from Table A.

Part 2 is where I am completely stuck. I have tried “list rows” and “get a row” and also using filters (if exists/not exists, if equal to not equal to) but I am unable to update existing rows (usually bad parameters message).

What is the correct sequence of modules to achieve this? Where can I enter the search term in “get a row”?

A kind person on the Integromat Facebook forum more or less confirmed that this is only really possible using the API call function, which is too over my skill level at the moment. I am going to try and find a way to make it work within ST itself.