Populating a form's text box based on that input in another text box

[Bigdaddrock]

I have a form that asks for the input of "Postal Code" in a Zip Code format using the mask (00000\-9999;;_).
There is another text box called TAXCODE that I would like to Autopopulate based on the first five digits of the postal code.
The source of the population is from the Table called TAXTABLE. This has two fields: ZIPCODE and TAXCODE.
Not all of the Postal Codes are found in the TAXTABLE. In those instances the form's TAXCODE should remain blank. Where the first five digits of Postal Code appear in the field ZIPCODE, I would like to autopopulate TAXCODE with the associated value found in the Table's TAXCODE field.
I envision using an AFTERUPDATE EVENT for the Postal Code field on the form. I also assume I need to incorporate an If statement that would leave the box blank if the record is not found, and a set value directive if it is found.
Can anyone help me with the appropriate language?
Again, the form's text boxes are Postal Code and TAXCODE.
The table's fields are ZIPCODE and TAXCODE.
Thanks for the support. I am running way over deadline on completing this project.

[Guido Geurs]

Please attach the XLS file in BYTES so we can see what You have and what You want exactly.
If possible, attach also an example of what the end-result must be

[Bigdaddrock]

Sorry about the miscommunication, this should have been in the MS ACCESS sector.
Thanks for the offer of assistance.