I'm new to Power BI, How can I write an IF statement using two differet tables and return N/A in the second table if conditions are met?

= IF VW-tdsls401100 (Qty B/O) = 0, else, ttdsls903100 (Est. Ship Date),N/A or blank

Tried creating a relationship between both tables. Both tables tdsls401100 & tdsls903100 are related by a primary key column. Looked on line to research how to set up the IF statement across two tables.