Beginner question: Add current card name as parameter to URL

Hello everyone,

I have just started to learn created PowerUps and have no real experience with JavaScript.
I want to have a simple PowerApp displayed in an iframe at the card back section. We want to have the first part of the card name as parameter to the URL, to have specific data shown by the PowerApp.

The card name looks something like this:

- 123456_descriptive_text

where “123456” is an identifier.

The code I’m using so far looks like this:

var Promise = TrelloPowerUp.Promise;
var GRAY_ICON = '';

  "card-back-section": function(t, options){
    return {
      title: 'Embedded PowerApps',
      icon: GRAY_ICON,
      content: {
        type: 'iframe',
        url: t.signUrl('[powerapps id]?tenantId=[tenantid]'),
        height: 500, // Max height is 1500.

I’m trying to achieve something similar like this:

 url: t.signUrl('[powerapps id]?tenantId=[tenantid]?SearchText='+**identifier**),

Since I have to trim down the card name string to the identifier, I would have to extract the card name in a way I can do some string manipulation.

Having spent two days and going through all that I could find on this topic with no viable solution in sight, I hope someone can help me out with this issue.