Custom field

Is it possible to put code into a custom field to make it act like a button.
Reason why I am asking is in New Zealand we have a tax table in which we can look up the pay amount and find amount of tax ,kiwi saver etc. If I could code a custom button to open the tax table on top of the manager screen that would save so much time. If I could and I was really cleaver I could code it to find the pay amount on open.
Now I do the payslip then open the tax table and find the amount and then go back and forth to find each amount taxed

I dont understand I can open a folder with HTML in a custom field but I can not open an excel file

This does not answer your question but I believe it could make the calculation easier than using the tax table … see

Yes that is a good calculator but it would make the job so much slower because you have to fill out all the details. With the tables you find the amount and just follow along to the different columns its quick