Inline: when the page *is* the form
An iframe inside the page makes sense in one case: when filling it in is why the person came. A «Contact us» page, an event signup, an order. There the form should be the first thing visible, not the last.
The code is an iframe with a fixed height. The one real precaution is not putting it in a narrow container: below 320 pixels wide the questions wrap three times and the form looks broken even when it isn't.
Popup and panel: when the form is an offer
If the page is about something else and the form is an invitation — «get in touch», «request a quote», «subscribe» — a button that opens a centred window costs zero space until someone clicks. The difference from an inline iframe isn't cosmetic: it's that the form can sit at the top of the page without taking it up.
The side panel is the same thing with a different animation, and works better for longer forms: it slides in from the right, uses the full height, and closing it doesn't lose your place on the page beneath.
The tab: once, across the whole site
A fixed tab on the right edge is how you keep a channel permanently available without adding it to every page by hand: paste the script into the layout and it appears everywhere. Use it for one thing only — usually feedback or contact — because two tabs on the same page are noise.
Careful on mobile: the tab takes the edge where some browsers put navigation gestures. If the site already has a chat widget or a floating button, pick one.
Two things to check afterwards
Cookies don't travel in a cross-site iframe. If you turned on «one response per person» without login, inside an embed that constraint becomes best-effort: someone who reloads can answer again. If it genuinely matters, you need required login — or the direct link instead of the iframe.
The credit at the bottom disappears by itself inside an embed (`?embed=1`), so the form doesn't look borrowed from another site. With Pro it goes from the direct link too.
Inline only if the page is the form; otherwise popup or panel, which sit at the top without taking up room.