I will _________ at eight o'clock, can you come later?

We use will, when:
* when we express beliefs about the future. 
Example: I think Brazil will win the World Cup.

* to mean want to or be willing to:
Example: I hope you will come to my party.

* to talk about offers and promises
Example: Mary will help with the cooking.

On the other hand, , 
**We use, will be with an -ing form for something happening before and after a specific time in the future.
example: They'll be waiting for you when you arrive.

- Similarly in this sentence , the right answer is: be working 
- The full sentence is: I will be working at eight o'clock, can you come later? 

অর্থাৎ যেহেতু এই বাক্যে ভবিষ্যতে কোন নির্দিষ্ট সময়ের আগে বা পরে কোনকিছু ঘটার কথা বলছে, সুতরাং এই ধরনের বাক্যে be working  বসে থাকে। 

Source: British council learning English.