Hi @Helgar ,
Do you want to set the priority in Create a task?
Could you please tell me what do you mean ‘In the form the priority they can choose from is a description; "Within a week", "within a month" or "When there is time".’?
The priority of Task have fixed four options

Do you mean your task bucket?

You can chose Bucket like this.

Or set Priority like this
Priority of the task. Valid range of values is between 0 and 10 (inclusive), with increasing value being lower priority (0 has the highest priority and 10 has the lowest priority). Currently, Planner interprets values 0 and 1 as "urgent", 2 and 3 and 4 as "important", 5, 6, and 7 as "medium", and 8, 9, and 10 as "low". Currently, Planner sets the value 1 for "urgent", 3 for "important", 5 for "medium", and 9 for "low".

Here is an article for your reference.
https://docs.microsoft.com/en-us/connectors/planner/#create-a-task-(preview)
Best Regards,
Zhi Chen