Can I choose a prefix to the transaction ID when communicating with Pay-Pal?

Hi,

Unfortunately, it is not possible to customize/insert any prefix on Paypal's default transaction ID. However, when you use Paypal Pro payment tool, you can pass custom data on Paypal Transaction. 

Example, you can insert a unique ID field to your form with the prefix you prefer and then set the unique ID field to be passed on the Paypal Transaction. Check the following visual guides.

Unique ID can be found under Widgets.

To set it on the Paypal Pro, check this.

Hope this helps!