You do not need a special Chinese IME to write a few Pinyin syllables with marks. You need a way to put ā, á, ǎ, à on the vowel.
Fastest option on this site
In the Pinyin Writer, type the syllable and a number: hao3 becomes hǎo. Use 5 when you want the dotted neutral-tone style the tool describes.
Windows
The US International keyboard and the built-in Pinyin IME can both produce marked vowels, but they take setup. For a short caption, the Writer is less fiddly than adding a keyboard layout.
macOS
Press and hold a vowel in many apps to see accent choices. That works for á and à more easily than for the third-tone caron (ǎ), which is why learners still like number input.
Android and iPhone
Press and hold vowels on the system keyboard. Coverage varies by keyboard app. Gboard and Apple's keyboard both offer some accents; the third tone may still be missing.
If you already have Chinese characters, skip typing marks and generate Pinyin from the text.