Feature summary (what you would like to be able to do and where):
Soon to be implemented into Special:CheckUser, Special:InvestigateBlock should allow options in a dropdown that can be chosen to either prefix the reason or be the reason for the block. This makes blocking faster as in the case of using generic block reasons the CheckUser can use the dropdown.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
CheckUser selects the "Abusing multiple accounts" dropdown option and then means they do not need to type this text when making the block. On enwiki, the CheckUser could use a custom dropdown option (definable through a MediaWiki interface page) to have the block reason as "{{CheckUserBlock}}".

Benefits (why should this be implemented?):
Would decrease the time spent typing for common block reasons.

Blockers
There is one blocker:

• T319517 - Being able to set the "selectandother" OOUI field as being required while still allowing the CheckUser to just use a dropdown option. Currently marking the field as required means the CheckUser must then add some content to the input field even if they choose an option from the dropdown.