Put the submission value into the block

I am helping my 10 yrs old son to build an app. The thing I want is make a form in a Screen1, and the value (Google Apps Script URL) to be stored into a block. See the attached image for clarification.
Can you please help me to make the form and the blocks workaround!

how the user input the value? by textbox?
Please elaborate your self clearly.

Yes, with textbox and a button. Or if you know any easier workaround, please tell me!
Thanks a lot for your help!

It is still not very clear as to what you want to do?

Why typing (pasting) a google apps script web app url to a textbox?

If you have more than one web app url, store these in a list for selection using a listpicker or listview, otherwise (just one web app url) hard code this into the app.

Every user that downloads the app has to have his own app script.
They need to create a Google Sheet along with an App Script on their Google account. Everyone will work only on their own sheet.

Have you seen this?

Which may make things a bit easier for your users.....

Yes, tried it but to mcuh complicated for 10 yrs users.
To simplify my request; I just want to save the submitted value (in this case the URL of the GAS) into the "Join" block.

Imagine not being the GAS Url. Let it be the child's name.
How can I save his name into the block and maintain it unchanged until he changes it again via the form.

Use a variable.