IndexNow on Squarespace: what to do instead
Squarespace has no IndexNow setting and does not let you upload the key file. So IndexNow is not possible on Squarespace.
IndexNow on Squarespace: Not available
With CrawlCoach
CrawlCoach can't do it on Squarespace either. Submit /sitemap.xml in Bing Webmaster Tools instead.
What IndexNow does, and what it does not
IndexNow tells Bing and other search engines, such as Yandex, Naver, Seznam, and Yep, that a page changed. Google does not use it. For Google, submit your sitemap in Search Console and fix the reasons Google gives for skipping pages.
For the Google side on Squarespace, see Squarespace pages not indexed.