As with all CK3 achievements, it depends on luck.
You can do this with any character. But I did this with King Alfonso VI of Leon in the 1066 start. He starts off with a lover (his sister) who is a landed vassal, and has great intrigue skill (which will make it easier to murder him). Soon after the start she will likely marry someone as that ai character will want to gain an alliance with you or your brothers. As soon as that happens plot to murder your him and try to get the chance to succeed as high to 95% as possible.
To make this quicker make sure that you have the following lifestyle skills unlocked on the Intrigue - Schemer tree as they will give you a higher success chance
- The Truth is relative (Prerequisite for the next 2)
- Swift Execution
- A Job done right