How to populate a Field based on selected Link's Child Table

Hi there,

My company has different Commission Rates for sales partners depending on whether a good is discounted. I have attempted to create a child table within Sales Partners Doctype to display “Promotional Code”, Promotional Type and Commission Rate (as shown below):


I then want my users to select the sales partner in the price List and the applicable Promotional Code if neccessary. I have linked the Sales Partner Doctype but I am lost on how to set up a select field to be populated with the Promotional Code available from the Child Table. Is there a way to do so?

Any help would be great thanks!

Please check the video.