Hi,
The idea is the create a link to an external application. Right now this is the table we are using with 2 fields (a lot more, but those are not relevant)
external_invoice_number
external_url
The external_invoice_number is a readonly field
The external_url is a hidden field
I thought of using the Calculation Type: Expression for the external_invoice_number field to create the link, but somehow it is not working (or my code is not correct)
![](https://uploads-eu-west-1.insided.com/thinkwise-en/attachment/d48a7bbb-739a-4f0c-817d-24a5cefdf595.png)
Why is this not working and what is the correct way to create a link? This is the code in the expression
case when t1.external_invoice_number not null and t1.external_url not null then
'<a href="' + t1.external_url +'">' + t1.external_invoice_number + '</a>'
when t1.external_invoice_number not null and t1.external_url is null then
t1.external_invoice_number
else
''
end