Can I pay for my form on monthly basis?

Are you referring to pay for your subscription as monthly basis?

I can see you are a free user and if you want to upgrade your subscription, you can pay it as a monthly basis.

When you choose to upgrade your subscription, you will get an option to select your Payment Schedule.

Please refer to this screenshot form Bronze plan:

Hope this is what you are looking for.

If you have any further query, please let us know.