An embedded upload form sits on a page of your own website, so people upload without ever leaving your site. It is the same form as the one on your share link, with the same fields, the same upload destination, and the same submissions. Embedding is available on the Business and Premium plans.
The form has to be published before an embedded copy will work. See Form statuses.

The embed code is JavaScript, built to load your form quickly. Paste the whole block, exactly as copied, at the point on the page where the form should appear.
On the Starter plan the Embed Code section is dimmed behind a padlock, above a line reading "Embed codes are available in Business and Premium plans."
Four switches sit above the embed code. Three of them are grouped under Show on the embedded form, and Transparent background sits on its own below them. All four start off.
Because all four start off, an embed code copied without touching them gives you the fields, the File Upload area, and the submit button on their own, on the form's usual background. That is usually what you want when the page around the form already carries your logo and explains what to send, and what is left still carries the form's colors, font, and styling. See Style your form.
Turn Logo, Form title, and Description on to carry the form's logo, title, and description into the embedded copy, the way they appear on the form's own share link.
Set the switches before you copy. Each one is written into the embed code at the moment you copy it rather than saved on the form, so all four are off again the next time you open the tab, and changing one later means copying the code again and replacing what you pasted.
Transparent background is for making the form read as part of the page rather than as a panel dropped onto it. The form's colors, fonts, and field styling stay exactly as you set them and only the background goes, so a form built in a dark style still looks dark against a dark page. Leave it off when the page behind the form is busy, or when the form needs to stand as its own block.
Preview opens an Embed Preview window showing the form as your switches have it, so you can check the result before touching your site.
Transparent background is the one switch Preview cannot show you. The window puts the app's own gray background behind the form rather than your page, so turning the switch on makes little visible difference here. Check that one on your own page instead.
The password screen and the CAPTCHA are both skipped in Preview, so a password-protected form shows its fields here and still asks for the password once it is embedded for real.

Paste the code into whatever your site builder calls a code element, at the spot where the form should appear, then save and publish that page.
Anything else that accepts raw HTML works the same way.
WordPress has one catch. The Custom HTML block works on self-hosted WordPress and on WordPress.com plans that include hosting features, which starts at their Business tier. WordPress.com's Free, Personal, and Premium plans strip script and iframe tags out, so neither the embed code nor the fallback code will show a form on any of them.
The embedded form sizes itself to fit. There is no height for you to set, and the space the form takes up on your page grows and shrinks with it as fields appear and files are added, so it never scrolls inside its own box.
Form not showing up on your site? below the buttons opens a second, fallback embed code. Some websites block the sizing script the embed code relies on, which stops the form from loading. The fallback code loads the form in an iframe instead, which does not need that script.
Without the sizing script the fallback cannot measure your form, so it needs a height given to it. Fallback height sets that height in pixels and starts at 1000. Set it taller than the form needs rather than shorter. A form taller than the height you set is cut off, while extra height shows as empty space below the form. Change the number first, then copy the fallback code, because the number is written into the code as you copy it.

Reach for the fallback only when the default code has not worked. The default code sizes itself to the form and pre-fills fields from query parameters on your page's address. The fallback code does neither.
The four switches apply to both codes. Set them before you copy either one.
An embedded form behaves almost exactly like the form on its own share link. Uploads are delivered to the same upload destination, submissions land in the same place, notifications send the same way, and file organization works the same. Four things are worth spelling out.