When email verification is enough
Plenty of sites label the step "verify your email" and send a one-time code or a confirmation link. If the form has an email field and no required phone field, a temporary inbox will complete it exactly like a real mailbox would, as long as the sender does not block disposable domains.
Paste the address from the widget, submit, and watch this tab. The code usually lands within seconds. Copy it back into the other site, finish the signup, and let the inbox expire.
What this cannot do
This is an email inbox, not a phone. It does not receive SMS text messages, so if a site insists on a mobile number for its one-time passcode, a temporary email will not help with that specific step.
It also cannot get past sites that require phone verification in addition to email. Use it for the email portion; if a phone number is mandatory, that part needs a real number.
Keep it to low-stakes accounts
Because the inbox is public and lasts five minutes, verify only accounts you do not need to recover later. Password resets, banking, and anything confidential should go to an email you control — anyone who knows the live address can read the code until it expires.
If nothing arrives, confirm the address, wait a few seconds, and check whether the site blocks disposable mail. When the timer ends, start a new inbox before requesting another code.