It depends on the software you are using, your hardware (PC or Mac), possibly your operating system, etc.
The Mac is easiest, simply hold the key that has accents (a, e i,o, u, in many languages; and there are lots more), the options will pop up, just chose the one you need.
For the PC there are numerous options. Do a google search for something like “insert accented letters in Chrome”.
The character map works for word processors. I have no experience for browser forms.
If you frequently type in a certain language other than English, install a foreign language or international keyboard. Do search for something like “international keyboard Windows 10,” and you’ll get the instructions. I’ve done this for Czech, which has lots of diacritical marks.
There are other options … extensions … cut and paste from a word processor document … alt codes … and the list goes on.
