[web2py] Adding extra form elements to SQLFORM

I am having trouble properly formatting my extra elements to match the rest of the form. This is a quote from the web2py book below. Can someone tell me what is exactly meant by "*The variable my_extra_element should be adapted to the formstyle*"? I am using bootsrap3_inline as formstyle, and

[web2py] Adding extra form elements to SQLFORM

I'm wondering about further usage of "a*dding extra form elements to SQLFORM", as written in the manual (link and code at the bottom). I have a table that works fine to insert records into, fully automated with SQLFORM. When I added an agree field to the SQLFORM, the form is still accepted upon