Getting markers for tooltips on the line chart

Hi guys!

I've tried to replicate the following solutions but with no luck:

BrianGrant 

https://community.powerbi.com/t5/Webinars-and-Video-Gallery/Annotating-Specific-Events-in-Line-Charts/m-p/151665

https://www.youtube.com/watch?v=jaici0AtX5Y&ab_channel=JasonDavidson

My data source is this:



I want to get a line chart with the total sales over time using the main calendar. On top of that i want to insert a date marker for every price change from the price changes table (ie. the date from that table). I've inserted "Starting Date" column on a tooltip and got this:



This is completely fine. HOWEVER, i want to get a marker for these points wherever there exists a Starting Date or better to say i want to transform an active tooltip into a marker. To do that i need to create a new masure with dax and insert it as a value and show it as a marker. The thing is that i cannot get the right expression.

The logic would be something like this:

if there exists a starting date give me a value of that starting date else BLANK()

Any ideas?