Microsoft has made it easier to customise “modern” SharePoint Online Lists and Libraries Forms natively.
But what do you do if you need to managing list is “classic” Form or the edit for option is just not available?
One option is to write a PowerShell script, see example below which will hide or show fields within New or Edit Forms.
This code and other scripts can be accessed in GitHub.
For more information on customising Modern SharePoint Forms, see these Microsoft articles: