LOD Expressions from Tableau

Hi,

I am trying to calculate the first purchase date of a given customer.

Each row has a customer ID and a purchase date, leading two a structure with multiple rows that have the same customer ID with different purchase dates.

In Tableau, I used the "Fixed" function, but this function does not seem to exist in Power BI.

I've also tried what this post mentions, but the expression used in this post returns the first purchase date for ALL customer ID's for me.

https://dataveld.com/2018/02/17/tableau-to-power-bi-fixed-lod-expressions/

Any help would be appreciated.

Thanks.