Start form without start process

Hello to all,
I need to start a process with a form. For example:
request for practice opening, if you do not finalize the request I don’t want that start a process instance.
I would like to show a form first, present in the bpmn, after which the process starts if the user continues after the above-mentioned form.

I noticed that it is possible in the modeler task to start a process with a form. If you put a form key on the startevent, it shows the form, then after clicking on start process you start it, and that’s exactly what I need. How does it work?

Thank you all for your help.

It’s been a few days without a response. It appears that you have a working solution, do you have a specific question?

Well you can get the form and also render it yourself via the forms REST api (/form-api/form/model) then on form submission, start the process with the submitted variables via the process api(/process-api/runtime/process-instances) REST api.