What I did is I copied the html code into the editor of a WP page like this:
<form id="form-wysija-nl-php-1" method="post" action="#wysija" class="widget_wysija form-valid-sub"><p class="wysija-instruct">To subscribe to our newsletter simply add your email.</p><p><input id="form-wysija-nl-php-1-wysija-to" class="wysija-email validate[required,custom[email]]" name="wysija[user][email]" type="text"><span class="wysija-p-firstname abs-req"><input defaultvalue="" id="form-wysija-nl-php-1-abs-firstname" value="" class="defaultlabels validated[abs][req]" name="wysija[user][abs][firstname]" type="text"></span><span class="wysija-p-lastname abs-req"><input defaultvalue="" id="form-wysija-nl-php-1-abs-lastname" value="" class="defaultlabels validated[abs][req]" name="wysija[user][abs][lastname]" type="text"></span><span class="wysija-p-email abs-req"><input defaultvalue="" id="form-wysija-nl-php-1-abs-email" value="" class="defaultlabels validated[abs][email]" name="wysija[user][abs][email]" type="text"></span><input class="wysija-submit wysija-submit-field" name="submit" value="Subscribe" type="submit"></p><input name="formid" value="form-wysija-nl-php-1" type="hidden">
<input name="action" value="save" type="hidden">
<input name="wysija[user_list][list_ids]" value="1" type="hidden">
<input name="message_success" value="You’ve successfully subscribed. Check your inbox now to confirm your subscription." type="hidden">
<input name="controller" value="subscribers" type="hidden"><input id="_wpnonce" name="_wpnonce" value="42e8b920d8" type="hidden"><input name="_wp_http_referer" value="/new_home/" type="hidden"><input value="1" name="wysija-page" type="hidden"><input value="9787924692" id="wysijax" type="hidden"></form>
On this page there was already this form which came from the generator from Wysija. This one:
<div class="wysija-register">YTo3OntzOjk6IndpZGdldF9pZCI7czoyMDoid3lzaWphLW5sLTEzNDA5MzE1ODUiO3M6NToibGlzdHMiO2E6MTp7aTowO3M6MToiMSI7fXM6MTA6Imxpc3RzX25hbWUiO2E6MTp7aToxO3M6MTc6Ik1pam4gZWVyc3RlIGxpanN0Ijt9czoxMjoiYXV0b3JlZ2lzdGVyIjtzOjE3OiJub3RfYXV0b19yZWdpc3RlciI7czoxMjoibGFiZWxzd2l0aGluIjtzOjEzOiJsYWJlbHNfd2l0aGluIjtzOjY6InN1Ym1pdCI7czoyOToiQWFubWVsZGVuIHZvb3IgZGUgbmlldXdzYnJpZWYiO3M6Nzoic3VjY2VzcyI7czo3MzoiVSBoZWVmdCB6aWNoIGluZ2VzY2hyZXZlbi4gQ2hlY2sgdXcgZW1haWwgb20gdXcgYWFubWVsZGluZyB0ZSBiZXZlc3RpZ2VuLiI7fQ==</div>
And also in the footer I placed this html code in a widget and it also works fine on this same page. So in this page three times the form, and all work well. See: https://www.reinasnaaicafe.nl/contact
But on the other pages is does not work although it’s inserted the same way with html on a page and in the footer.. For instance: https://www.reinasnaaicafe.nl/feestje (see footer).