how to pre-populate form when a drop down box is selected ?

I would like to have a form designed so when a certain name in a drop down box is selected

the email address, team name and player jersey number are populated on the form which save the manual selection  

Below is a form derived from one of your forms I presume you're working on right now.

https://form.jotform.com/63034755477967

If you select "9 Kosta Agathopoulos" from the "Name - If name doesn't appear Click on No Name" field, the Squad, Jersey Number - If not known leave Blank, and Email fields will also be filled.

Same if you select "9 ETHAN ALAS" from the "Name - If name doesn't appear Click on No Name" field.

You will be able to do that with the help of Update / Calculate conditional logic.

https://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic

Below is how the condition is done.

I hope this helps. Please let us know if you need any further assistance. Thank you.