If you are embedding a form in a context where your users are logged in, asking them to provide their email addresses might be a little awkward. Instead of doing that, you can prepopulate certain fields by modifying the URL in the embed script. This works especially well with hidden fields in Reform (fields that aren't rendered in the form).
To prepopulate a form field, you need to modify the URL to contain the ID of the field and the value you want to prepopulate with. Let's say the field ID is "331149da-a744-4212-99eb-b5c899f86d31" and you want it to be prefilled with the text "John Doe", the URL looks like this:
The "%20" is how a space looks in a URL when it's URL encoded.
You can get the ID for a field by clicking the ⚙️ icon in the form builder: